1/* SPDX-License-Identifier: GPL-2.0+ */
   2/*
   3 * Task-based RCU implementations.
   4 *
   5 * Copyright (C) 2020 Paul E. McKenney
   6 */
   7
   8#ifdef CONFIG_TASKS_RCU_GENERIC
   9#include "rcu_segcblist.h"
  10
  11////////////////////////////////////////////////////////////////////////
  12//
  13// Generic data structures.
  14
  15struct rcu_tasks;
  16typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
  17typedef void (*pregp_func_t)(struct list_head *hop);
  18typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
  19typedef void (*postscan_func_t)(struct list_head *hop);
  20typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
  21typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
  22
  23/**
  24 * struct rcu_tasks_percpu - Per-CPU component of definition for a Tasks-RCU-like mechanism.
  25 * @cblist: Callback list.
  26 * @lock: Lock protecting per-CPU callback list.
  27 * @rtp_jiffies: Jiffies counter value for statistics.
  28 * @lazy_timer: Timer to unlazify callbacks.
  29 * @urgent_gp: Number of additional non-lazy grace periods.
  30 * @rtp_n_lock_retries: Rough lock-contention statistic.
  31 * @rtp_work: Work queue for invoking callbacks.
  32 * @rtp_irq_work: IRQ work queue for deferred wakeups.
  33 * @barrier_q_head: RCU callback for barrier operation.
  34 * @rtp_blkd_tasks: List of tasks blocked as readers.
  35 * @rtp_exit_list: List of tasks in the latter portion of do_exit().
  36 * @cpu: CPU number corresponding to this entry.
  37 * @index: Index of this CPU in rtpcp_array of the rcu_tasks structure.
  38 * @rtpp: Pointer to the rcu_tasks structure.
  39 */
  40struct rcu_tasks_percpu {
  41	struct rcu_segcblist cblist;
  42	raw_spinlock_t __private lock;
  43	unsigned long rtp_jiffies;
  44	unsigned long rtp_n_lock_retries;
  45	struct timer_list lazy_timer;
  46	unsigned int urgent_gp;
  47	struct work_struct rtp_work;
  48	struct irq_work rtp_irq_work;
  49	struct rcu_head barrier_q_head;
  50	struct list_head rtp_blkd_tasks;
  51	struct list_head rtp_exit_list;
  52	int cpu;
  53	int index;
  54	struct rcu_tasks *rtpp;
  55};
  56
  57/**
  58 * struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
  59 * @cbs_wait: RCU wait allowing a new callback to get kthread's attention.
  60 * @cbs_gbl_lock: Lock protecting callback list.
  61 * @tasks_gp_mutex: Mutex protecting grace period, needed during mid-boot dead zone.
  62 * @gp_func: This flavor's grace-period-wait function.
  63 * @gp_state: Grace period's most recent state transition (debugging).
  64 * @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
  65 * @init_fract: Initial backoff sleep interval.
  66 * @gp_jiffies: Time of last @gp_state transition.
  67 * @gp_start: Most recent grace-period start in jiffies.
  68 * @tasks_gp_seq: Number of grace periods completed since boot in upper bits.
  69 * @n_ipis: Number of IPIs sent to encourage grace periods to end.
  70 * @n_ipis_fails: Number of IPI-send failures.
  71 * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
  72 * @lazy_jiffies: Number of jiffies to allow callbacks to be lazy.
  73 * @pregp_func: This flavor's pre-grace-period function (optional).
  74 * @pertask_func: This flavor's per-task scan function (optional).
  75 * @postscan_func: This flavor's post-task scan function (optional).
  76 * @holdouts_func: This flavor's holdout-list scan function (optional).
  77 * @postgp_func: This flavor's post-grace-period function (optional).
  78 * @call_func: This flavor's call_rcu()-equivalent function.
  79 * @wait_state: Task state for synchronous grace-period waits (default TASK_UNINTERRUPTIBLE).
  80 * @rtpcpu: This flavor's rcu_tasks_percpu structure.
  81 * @rtpcp_array: Array of pointers to rcu_tasks_percpu structure of CPUs in cpu_possible_mask.
  82 * @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
  83 * @percpu_enqueue_lim: Number of per-CPU callback queues in use for enqueuing.
  84 * @percpu_dequeue_lim: Number of per-CPU callback queues in use for dequeuing.
  85 * @percpu_dequeue_gpseq: RCU grace-period number to propagate enqueue limit to dequeuers.
  86 * @barrier_q_mutex: Serialize barrier operations.
  87 * @barrier_q_count: Number of queues being waited on.
  88 * @barrier_q_completion: Barrier wait/wakeup mechanism.
  89 * @barrier_q_seq: Sequence number for barrier operations.
  90 * @barrier_q_start: Most recent barrier start in jiffies.
  91 * @name: This flavor's textual name.
  92 * @kname: This flavor's kthread name.
  93 */
  94struct rcu_tasks {
  95	struct rcuwait cbs_wait;
  96	raw_spinlock_t cbs_gbl_lock;
  97	struct mutex tasks_gp_mutex;
 
  98	int gp_state;
  99	int gp_sleep;
 100	int init_fract;
 101	unsigned long gp_jiffies;
 102	unsigned long gp_start;
 103	unsigned long tasks_gp_seq;
 104	unsigned long n_ipis;
 105	unsigned long n_ipis_fails;
 106	struct task_struct *kthread_ptr;
 107	unsigned long lazy_jiffies;
 108	rcu_tasks_gp_func_t gp_func;
 109	pregp_func_t pregp_func;
 110	pertask_func_t pertask_func;
 111	postscan_func_t postscan_func;
 112	holdouts_func_t holdouts_func;
 113	postgp_func_t postgp_func;
 114	call_rcu_func_t call_func;
 115	unsigned int wait_state;
 116	struct rcu_tasks_percpu __percpu *rtpcpu;
 117	struct rcu_tasks_percpu **rtpcp_array;
 118	int percpu_enqueue_shift;
 119	int percpu_enqueue_lim;
 120	int percpu_dequeue_lim;
 121	unsigned long percpu_dequeue_gpseq;
 122	struct mutex barrier_q_mutex;
 123	atomic_t barrier_q_count;
 124	struct completion barrier_q_completion;
 125	unsigned long barrier_q_seq;
 126	unsigned long barrier_q_start;
 127	char *name;
 128	char *kname;
 129};
 130
 131static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
 132
 133#define DEFINE_RCU_TASKS(rt_name, gp, call, n)						\
 134static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = {			\
 135	.lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock),		\
 136	.rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup),			\
 137};											\
 138static struct rcu_tasks rt_name =							\
 139{											\
 140	.cbs_wait = __RCUWAIT_INITIALIZER(rt_name.wait),				\
 141	.cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock),			\
 142	.tasks_gp_mutex = __MUTEX_INITIALIZER(rt_name.tasks_gp_mutex),			\
 143	.gp_func = gp,									\
 144	.call_func = call,								\
 145	.wait_state = TASK_UNINTERRUPTIBLE,						\
 146	.rtpcpu = &rt_name ## __percpu,							\
 147	.lazy_jiffies = DIV_ROUND_UP(HZ, 4),						\
 148	.name = n,									\
 149	.percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS),				\
 150	.percpu_enqueue_lim = 1,							\
 151	.percpu_dequeue_lim = 1,							\
 152	.barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex),		\
 153	.barrier_q_seq = (0UL - 50UL) << RCU_SEQ_CTR_SHIFT,				\
 154	.kname = #rt_name,								\
 155}
 156
 157#ifdef CONFIG_TASKS_RCU
 158
 159/* Report delay of scan exiting tasklist in rcu_tasks_postscan(). */
 160static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
 161static DEFINE_TIMER(tasks_rcu_exit_srcu_stall_timer, tasks_rcu_exit_srcu_stall);
 162#endif
 163
 164/* Avoid IPIing CPUs early in the grace period. */
 165#define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0)
 166static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
 167module_param(rcu_task_ipi_delay, int, 0644);
 168
 169/* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
 170#define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
 171#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
 172static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
 173module_param(rcu_task_stall_timeout, int, 0644);
 174#define RCU_TASK_STALL_INFO (HZ * 10)
 175static int rcu_task_stall_info __read_mostly = RCU_TASK_STALL_INFO;
 176module_param(rcu_task_stall_info, int, 0644);
 177static int rcu_task_stall_info_mult __read_mostly = 3;
 178module_param(rcu_task_stall_info_mult, int, 0444);
 179
 180static int rcu_task_enqueue_lim __read_mostly = -1;
 181module_param(rcu_task_enqueue_lim, int, 0444);
 182
 183static bool rcu_task_cb_adjust;
 184static int rcu_task_contend_lim __read_mostly = 100;
 185module_param(rcu_task_contend_lim, int, 0444);
 186static int rcu_task_collapse_lim __read_mostly = 10;
 187module_param(rcu_task_collapse_lim, int, 0444);
 188static int rcu_task_lazy_lim __read_mostly = 32;
 189module_param(rcu_task_lazy_lim, int, 0444);
 190
 191static int rcu_task_cpu_ids;
 192
 193/* RCU tasks grace-period state for debugging. */
 194#define RTGS_INIT		 0
 195#define RTGS_WAIT_WAIT_CBS	 1
 196#define RTGS_WAIT_GP		 2
 197#define RTGS_PRE_WAIT_GP	 3
 198#define RTGS_SCAN_TASKLIST	 4
 199#define RTGS_POST_SCAN_TASKLIST	 5
 200#define RTGS_WAIT_SCAN_HOLDOUTS	 6
 201#define RTGS_SCAN_HOLDOUTS	 7
 202#define RTGS_POST_GP		 8
 203#define RTGS_WAIT_READERS	 9
 204#define RTGS_INVOKE_CBS		10
 205#define RTGS_WAIT_CBS		11
 206#ifndef CONFIG_TINY_RCU
 207static const char * const rcu_tasks_gp_state_names[] = {
 208	"RTGS_INIT",
 209	"RTGS_WAIT_WAIT_CBS",
 210	"RTGS_WAIT_GP",
 211	"RTGS_PRE_WAIT_GP",
 212	"RTGS_SCAN_TASKLIST",
 213	"RTGS_POST_SCAN_TASKLIST",
 214	"RTGS_WAIT_SCAN_HOLDOUTS",
 215	"RTGS_SCAN_HOLDOUTS",
 216	"RTGS_POST_GP",
 217	"RTGS_WAIT_READERS",
 218	"RTGS_INVOKE_CBS",
 219	"RTGS_WAIT_CBS",
 220};
 221#endif /* #ifndef CONFIG_TINY_RCU */
 222
 223////////////////////////////////////////////////////////////////////////
 224//
 225// Generic code.
 226
 227static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp);
 228
 229/* Record grace-period phase and time. */
 230static void set_tasks_gp_state(struct rcu_tasks *rtp, int newstate)
 231{
 232	rtp->gp_state = newstate;
 233	rtp->gp_jiffies = jiffies;
 234}
 235
 236#ifndef CONFIG_TINY_RCU
 237/* Return state name. */
 238static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
 239{
 240	int i = data_race(rtp->gp_state); // Let KCSAN detect update races
 241	int j = READ_ONCE(i); // Prevent the compiler from reading twice
 242
 243	if (j >= ARRAY_SIZE(rcu_tasks_gp_state_names))
 244		return "???";
 245	return rcu_tasks_gp_state_names[j];
 246}
 247#endif /* #ifndef CONFIG_TINY_RCU */
 248
 249// Initialize per-CPU callback lists for the specified flavor of
 250// Tasks RCU.  Do not enqueue callbacks before this function is invoked.
 251static void cblist_init_generic(struct rcu_tasks *rtp)
 252{
 253	int cpu;
 254	int lim;
 255	int shift;
 256	int maxcpu;
 257	int index = 0;
 258
 259	if (rcu_task_enqueue_lim < 0) {
 260		rcu_task_enqueue_lim = 1;
 261		rcu_task_cb_adjust = true;
 262	} else if (rcu_task_enqueue_lim == 0) {
 263		rcu_task_enqueue_lim = 1;
 264	}
 265	lim = rcu_task_enqueue_lim;
 266
 267	rtp->rtpcp_array = kcalloc(num_possible_cpus(), sizeof(struct rcu_tasks_percpu *), GFP_KERNEL);
 268	BUG_ON(!rtp->rtpcp_array);
 269
 270	for_each_possible_cpu(cpu) {
 271		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 272
 273		WARN_ON_ONCE(!rtpcp);
 274		if (cpu)
 275			raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
 276		if (rcu_segcblist_empty(&rtpcp->cblist))
 277			rcu_segcblist_init(&rtpcp->cblist);
 278		INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
 279		rtpcp->cpu = cpu;
 280		rtpcp->rtpp = rtp;
 281		rtpcp->index = index;
 282		rtp->rtpcp_array[index] = rtpcp;
 283		index++;
 284		if (!rtpcp->rtp_blkd_tasks.next)
 285			INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
 286		if (!rtpcp->rtp_exit_list.next)
 287			INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
 288		rtpcp->barrier_q_head.next = &rtpcp->barrier_q_head;
 289		maxcpu = cpu;
 290	}
 291
 292	rcu_task_cpu_ids = maxcpu + 1;
 293	if (lim > rcu_task_cpu_ids)
 294		lim = rcu_task_cpu_ids;
 295	shift = ilog2(rcu_task_cpu_ids / lim);
 296	if (((rcu_task_cpu_ids - 1) >> shift) >= lim)
 297		shift++;
 298	WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
 299	WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
 300	smp_store_release(&rtp->percpu_enqueue_lim, lim);
 301
 302	pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d rcu_task_cpu_ids=%d.\n",
 303			rtp->name, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim),
 304			rcu_task_cb_adjust, rcu_task_cpu_ids);
 305}
 306
 307// Compute wakeup time for lazy callback timer.
 308static unsigned long rcu_tasks_lazy_time(struct rcu_tasks *rtp)
 309{
 310	return jiffies + rtp->lazy_jiffies;
 311}
 312
 313// Timer handler that unlazifies lazy callbacks.
 314static void call_rcu_tasks_generic_timer(struct timer_list *tlp)
 315{
 316	unsigned long flags;
 317	bool needwake = false;
 318	struct rcu_tasks *rtp;
 319	struct rcu_tasks_percpu *rtpcp = from_timer(rtpcp, tlp, lazy_timer);
 320
 321	rtp = rtpcp->rtpp;
 322	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 323	if (!rcu_segcblist_empty(&rtpcp->cblist) && rtp->lazy_jiffies) {
 324		if (!rtpcp->urgent_gp)
 325			rtpcp->urgent_gp = 1;
 326		needwake = true;
 327		mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
 328	}
 329	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 330	if (needwake)
 331		rcuwait_wake_up(&rtp->cbs_wait);
 332}
 333
 334// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
 335static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
 336{
 337	struct rcu_tasks *rtp;
 338	struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);
 339
 340	rtp = rtpcp->rtpp;
 341	rcuwait_wake_up(&rtp->cbs_wait);
 342}
 343
 344// Enqueue a callback for the specified flavor of Tasks RCU.
 345static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
 346				   struct rcu_tasks *rtp)
 347{
 348	int chosen_cpu;
 349	unsigned long flags;
 350	bool havekthread = smp_load_acquire(&rtp->kthread_ptr);
 351	int ideal_cpu;
 352	unsigned long j;
 353	bool needadjust = false;
 354	bool needwake;
 355	struct rcu_tasks_percpu *rtpcp;
 356
 357	rhp->next = NULL;
 358	rhp->func = func;
 359	local_irq_save(flags);
 360	rcu_read_lock();
 361	ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
 362	chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_possible_mask);
 363	WARN_ON_ONCE(chosen_cpu >= rcu_task_cpu_ids);
 364	rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
 365	if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
 366		raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
 367		j = jiffies;
 368		if (rtpcp->rtp_jiffies != j) {
 369			rtpcp->rtp_jiffies = j;
 370			rtpcp->rtp_n_lock_retries = 0;
 371		}
 372		if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
 373		    READ_ONCE(rtp->percpu_enqueue_lim) != rcu_task_cpu_ids)
 374			needadjust = true;  // Defer adjustment to avoid deadlock.
 375	}
 376	// Queuing callbacks before initialization not yet supported.
 377	if (WARN_ON_ONCE(!rcu_segcblist_is_enabled(&rtpcp->cblist)))
 378		rcu_segcblist_init(&rtpcp->cblist);
 379	needwake = (func == wakeme_after_rcu) ||
 380		   (rcu_segcblist_n_cbs(&rtpcp->cblist) == rcu_task_lazy_lim);
 381	if (havekthread && !needwake && !timer_pending(&rtpcp->lazy_timer)) {
 382		if (rtp->lazy_jiffies)
 383			mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
 384		else
 385			needwake = rcu_segcblist_empty(&rtpcp->cblist);
 386	}
 387	if (needwake)
 388		rtpcp->urgent_gp = 3;
 389	rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
 390	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 391	if (unlikely(needadjust)) {
 392		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
 393		if (rtp->percpu_enqueue_lim != rcu_task_cpu_ids) {
 394			WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
 395			WRITE_ONCE(rtp->percpu_dequeue_lim, rcu_task_cpu_ids);
 396			smp_store_release(&rtp->percpu_enqueue_lim, rcu_task_cpu_ids);
 397			pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
 398		}
 399		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
 400	}
 401	rcu_read_unlock();
 402	/* We can't create the thread unless interrupts are enabled. */
 403	if (needwake && READ_ONCE(rtp->kthread_ptr))
 404		irq_work_queue(&rtpcp->rtp_irq_work);
 405}
 406
 407// RCU callback function for rcu_barrier_tasks_generic().
 408static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
 409{
 410	struct rcu_tasks *rtp;
 411	struct rcu_tasks_percpu *rtpcp;
 412
 413	rhp->next = rhp; // Mark the callback as having been invoked.
 414	rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
 415	rtp = rtpcp->rtpp;
 416	if (atomic_dec_and_test(&rtp->barrier_q_count))
 417		complete(&rtp->barrier_q_completion);
 418}
 419
 420// Wait for all in-flight callbacks for the specified RCU Tasks flavor.
 421// Operates in a manner similar to rcu_barrier().
 422static void __maybe_unused rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
 423{
 424	int cpu;
 425	unsigned long flags;
 426	struct rcu_tasks_percpu *rtpcp;
 427	unsigned long s = rcu_seq_snap(&rtp->barrier_q_seq);
 428
 429	mutex_lock(&rtp->barrier_q_mutex);
 430	if (rcu_seq_done(&rtp->barrier_q_seq, s)) {
 431		smp_mb();
 432		mutex_unlock(&rtp->barrier_q_mutex);
 433		return;
 434	}
 435	rtp->barrier_q_start = jiffies;
 436	rcu_seq_start(&rtp->barrier_q_seq);
 437	init_completion(&rtp->barrier_q_completion);
 438	atomic_set(&rtp->barrier_q_count, 2);
 439	for_each_possible_cpu(cpu) {
 440		if (cpu >= smp_load_acquire(&rtp->percpu_dequeue_lim))
 441			break;
 442		rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 443		rtpcp->barrier_q_head.func = rcu_barrier_tasks_generic_cb;
 444		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 445		if (rcu_segcblist_entrain(&rtpcp->cblist, &rtpcp->barrier_q_head))
 446			atomic_inc(&rtp->barrier_q_count);
 447		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 448	}
 449	if (atomic_sub_and_test(2, &rtp->barrier_q_count))
 450		complete(&rtp->barrier_q_completion);
 451	wait_for_completion(&rtp->barrier_q_completion);
 452	rcu_seq_end(&rtp->barrier_q_seq);
 453	mutex_unlock(&rtp->barrier_q_mutex);
 454}
 455
 456// Advance callbacks and indicate whether either a grace period or
 457// callback invocation is needed.
 458static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 459{
 460	int cpu;
 461	int dequeue_limit;
 462	unsigned long flags;
 463	bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
 464	long n;
 465	long ncbs = 0;
 466	long ncbsnz = 0;
 467	int needgpcb = 0;
 468
 469	dequeue_limit = smp_load_acquire(&rtp->percpu_dequeue_lim);
 470	for (cpu = 0; cpu < dequeue_limit; cpu++) {
 471		if (!cpu_possible(cpu))
 472			continue;
 473		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 474
 475		/* Advance and accelerate any new callbacks. */
 476		if (!rcu_segcblist_n_cbs(&rtpcp->cblist))
 477			continue;
 478		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 479		// Should we shrink down to a single callback queue?
 480		n = rcu_segcblist_n_cbs(&rtpcp->cblist);
 481		if (n) {
 482			ncbs += n;
 483			if (cpu > 0)
 484				ncbsnz += n;
 485		}
 486		rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
 487		(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
 488		if (rtpcp->urgent_gp > 0 && rcu_segcblist_pend_cbs(&rtpcp->cblist)) {
 489			if (rtp->lazy_jiffies)
 490				rtpcp->urgent_gp--;
 491			needgpcb |= 0x3;
 492		} else if (rcu_segcblist_empty(&rtpcp->cblist)) {
 493			rtpcp->urgent_gp = 0;
 494		}
 495		if (rcu_segcblist_ready_cbs(&rtpcp->cblist))
 496			needgpcb |= 0x1;
 497		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 498	}
 499
 500	// Shrink down to a single callback queue if appropriate.
 501	// This is done in two stages: (1) If there are no more than
 502	// rcu_task_collapse_lim callbacks on CPU 0 and none on any other
 503	// CPU, limit enqueueing to CPU 0.  (2) After an RCU grace period,
 504	// if there has not been an increase in callbacks, limit dequeuing
 505	// to CPU 0.  Note the matching RCU read-side critical section in
 506	// call_rcu_tasks_generic().
 507	if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
 508		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
 509		if (rtp->percpu_enqueue_lim > 1) {
 510			WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(rcu_task_cpu_ids));
 511			smp_store_release(&rtp->percpu_enqueue_lim, 1);
 512			rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
 513			gpdone = false;
 514			pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
 515		}
 516		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
 517	}
 518	if (rcu_task_cb_adjust && !ncbsnz && gpdone) {
 519		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
 520		if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
 521			WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
 522			pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
 523		}
 524		if (rtp->percpu_dequeue_lim == 1) {
 525			for (cpu = rtp->percpu_dequeue_lim; cpu < rcu_task_cpu_ids; cpu++) {
 526				if (!cpu_possible(cpu))
 527					continue;
 528				struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 529
 530				WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
 531			}
 532		}
 533		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
 534	}
 535
 536	return needgpcb;
 537}
 538
 539// Advance callbacks and invoke any that are ready.
 540static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
 541{
 542	int cpuwq;
 543	unsigned long flags;
 544	int len;
 545	int index;
 546	struct rcu_head *rhp;
 547	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
 548	struct rcu_tasks_percpu *rtpcp_next;
 549
 550	index = rtpcp->index * 2 + 1;
 551	if (index < num_possible_cpus()) {
 552		rtpcp_next = rtp->rtpcp_array[index];
 553		if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
 554			cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
 555			queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
 556			index++;
 557			if (index < num_possible_cpus()) {
 558				rtpcp_next = rtp->rtpcp_array[index];
 559				if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
 560					cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
 561					queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
 562				}
 563			}
 564		}
 565	}
 566
 567	if (rcu_segcblist_empty(&rtpcp->cblist))
 568		return;
 569	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 570	rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
 571	rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
 572	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 573	len = rcl.len;
 574	for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
 575		debug_rcu_head_callback(rhp);
 576		local_bh_disable();
 577		rhp->func(rhp);
 578		local_bh_enable();
 579		cond_resched();
 580	}
 581	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 582	rcu_segcblist_add_len(&rtpcp->cblist, -len);
 583	(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
 584	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 585}
 586
 587// Workqueue flood to advance callbacks and invoke any that are ready.
 588static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
 589{
 590	struct rcu_tasks *rtp;
 591	struct rcu_tasks_percpu *rtpcp = container_of(wp, struct rcu_tasks_percpu, rtp_work);
 592
 593	rtp = rtpcp->rtpp;
 594	rcu_tasks_invoke_cbs(rtp, rtpcp);
 595}
 596
 597// Wait for one grace period.
 598static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
 599{
 600	int needgpcb;
 601
 602	mutex_lock(&rtp->tasks_gp_mutex);
 603
 604	// If there were none, wait a bit and start over.
 605	if (unlikely(midboot)) {
 606		needgpcb = 0x2;
 607	} else {
 608		mutex_unlock(&rtp->tasks_gp_mutex);
 609		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
 610		rcuwait_wait_event(&rtp->cbs_wait,
 611				   (needgpcb = rcu_tasks_need_gpcb(rtp)),
 612				   TASK_IDLE);
 613		mutex_lock(&rtp->tasks_gp_mutex);
 614	}
 615
 616	if (needgpcb & 0x2) {
 617		// Wait for one grace period.
 618		set_tasks_gp_state(rtp, RTGS_WAIT_GP);
 619		rtp->gp_start = jiffies;
 620		rcu_seq_start(&rtp->tasks_gp_seq);
 621		rtp->gp_func(rtp);
 622		rcu_seq_end(&rtp->tasks_gp_seq);
 623	}
 624
 625	// Invoke callbacks.
 626	set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
 627	rcu_tasks_invoke_cbs(rtp, per_cpu_ptr(rtp->rtpcpu, 0));
 628	mutex_unlock(&rtp->tasks_gp_mutex);
 629}
 630
 631// RCU-tasks kthread that detects grace periods and invokes callbacks.
 632static int __noreturn rcu_tasks_kthread(void *arg)
 633{
 634	int cpu;
 
 
 635	struct rcu_tasks *rtp = arg;
 636
 637	for_each_possible_cpu(cpu) {
 638		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 639
 640		timer_setup(&rtpcp->lazy_timer, call_rcu_tasks_generic_timer, 0);
 641		rtpcp->urgent_gp = 1;
 642	}
 643
 644	/* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
 645	housekeeping_affine(current, HK_TYPE_RCU);
 646	smp_store_release(&rtp->kthread_ptr, current); // Let GPs start!
 647
 648	/*
 649	 * Each pass through the following loop makes one check for
 650	 * newly arrived callbacks, and, if there are some, waits for
 651	 * one RCU-tasks grace period and then invokes the callbacks.
 652	 * This loop is terminated by the system going down.  ;-)
 653	 */
 654	for (;;) {
 655		// Wait for one grace period and invoke any callbacks
 656		// that are ready.
 657		rcu_tasks_one_gp(rtp, false);
 658
 659		// Paranoid sleep to keep this from entering a tight loop.
 660		schedule_timeout_idle(rtp->gp_sleep);
 661	}
 662}
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 663
 664// Wait for a grace period for the specified flavor of Tasks RCU.
 665static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
 666{
 667	/* Complain if the scheduler has not started.  */
 668	if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
 669			 "synchronize_%s() called too soon", rtp->name))
 670		return;
 671
 672	// If the grace-period kthread is running, use it.
 673	if (READ_ONCE(rtp->kthread_ptr)) {
 674		wait_rcu_gp_state(rtp->wait_state, rtp->call_func);
 675		return;
 
 
 
 
 
 
 
 
 
 
 676	}
 677	rcu_tasks_one_gp(rtp, true);
 678}
 679
 680/* Spawn RCU-tasks grace-period kthread. */
 681static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
 682{
 683	struct task_struct *t;
 684
 685	t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
 686	if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
 687		return;
 688	smp_mb(); /* Ensure others see full kthread. */
 689}
 690
 691#ifndef CONFIG_TINY_RCU
 692
 693/*
 694 * Print any non-default Tasks RCU settings.
 695 */
 696static void __init rcu_tasks_bootup_oddness(void)
 697{
 698#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
 699	int rtsimc;
 700
 701	if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
 702		pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
 703	rtsimc = clamp(rcu_task_stall_info_mult, 1, 10);
 704	if (rtsimc != rcu_task_stall_info_mult) {
 705		pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc);
 706		rcu_task_stall_info_mult = rtsimc;
 707	}
 708#endif /* #ifdef CONFIG_TASKS_RCU */
 709#ifdef CONFIG_TASKS_RCU
 710	pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
 711#endif /* #ifdef CONFIG_TASKS_RCU */
 712#ifdef CONFIG_TASKS_RUDE_RCU
 713	pr_info("\tRude variant of Tasks RCU enabled.\n");
 714#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
 715#ifdef CONFIG_TASKS_TRACE_RCU
 716	pr_info("\tTracing variant of Tasks RCU enabled.\n");
 717#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
 718}
 719
 
 720
 
 721/* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
 722static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
 723{
 724	int cpu;
 725	bool havecbs = false;
 726	bool haveurgent = false;
 727	bool haveurgentcbs = false;
 728
 729	for_each_possible_cpu(cpu) {
 730		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 731
 732		if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)))
 733			havecbs = true;
 734		if (data_race(rtpcp->urgent_gp))
 735			haveurgent = true;
 736		if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)) && data_race(rtpcp->urgent_gp))
 737			haveurgentcbs = true;
 738		if (havecbs && haveurgent && haveurgentcbs)
 739			break;
 740	}
 741	pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c%c%c l:%lu %s\n",
 742		rtp->kname,
 743		tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
 744		jiffies - data_race(rtp->gp_jiffies),
 745		data_race(rcu_seq_current(&rtp->tasks_gp_seq)),
 746		data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
 747		".k"[!!data_race(rtp->kthread_ptr)],
 748		".C"[havecbs],
 749		".u"[haveurgent],
 750		".U"[haveurgentcbs],
 751		rtp->lazy_jiffies,
 752		s);
 753}
 754
 755/* Dump out more rcutorture-relevant state common to all RCU-tasks flavors. */
 756static void rcu_tasks_torture_stats_print_generic(struct rcu_tasks *rtp, char *tt,
 757						  char *tf, char *tst)
 758{
 759	cpumask_var_t cm;
 760	int cpu;
 761	bool gotcb = false;
 762	unsigned long j = jiffies;
 763
 764	pr_alert("%s%s Tasks%s RCU g%ld gp_start %lu gp_jiffies %lu gp_state %d (%s).\n",
 765		 tt, tf, tst, data_race(rtp->tasks_gp_seq),
 766		 j - data_race(rtp->gp_start), j - data_race(rtp->gp_jiffies),
 767		 data_race(rtp->gp_state), tasks_gp_state_getname(rtp));
 768	pr_alert("\tEnqueue shift %d limit %d Dequeue limit %d gpseq %lu.\n",
 769		 data_race(rtp->percpu_enqueue_shift),
 770		 data_race(rtp->percpu_enqueue_lim),
 771		 data_race(rtp->percpu_dequeue_lim),
 772		 data_race(rtp->percpu_dequeue_gpseq));
 773	(void)zalloc_cpumask_var(&cm, GFP_KERNEL);
 774	pr_alert("\tCallback counts:");
 775	for_each_possible_cpu(cpu) {
 776		long n;
 777		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 778
 779		if (cpumask_available(cm) && !rcu_barrier_cb_is_done(&rtpcp->barrier_q_head))
 780			cpumask_set_cpu(cpu, cm);
 781		n = rcu_segcblist_n_cbs(&rtpcp->cblist);
 782		if (!n)
 783			continue;
 784		pr_cont(" %d:%ld", cpu, n);
 785		gotcb = true;
 786	}
 787	if (gotcb)
 788		pr_cont(".\n");
 789	else
 790		pr_cont(" (none).\n");
 791	pr_alert("\tBarrier seq %lu start %lu count %d holdout CPUs ",
 792		 data_race(rtp->barrier_q_seq), j - data_race(rtp->barrier_q_start),
 793		 atomic_read(&rtp->barrier_q_count));
 794	if (cpumask_available(cm) && !cpumask_empty(cm))
 795		pr_cont(" %*pbl.\n", cpumask_pr_args(cm));
 796	else
 797		pr_cont("(none).\n");
 798	free_cpumask_var(cm);
 799}
 800
 801#endif // #ifndef CONFIG_TINY_RCU
 802
 803static void exit_tasks_rcu_finish_trace(struct task_struct *t);
 804
 805#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
 806
 807////////////////////////////////////////////////////////////////////////
 808//
 809// Shared code between task-list-scanning variants of Tasks RCU.
 810
 811/* Wait for one RCU-tasks grace period. */
 812static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 813{
 814	struct task_struct *g;
 815	int fract;
 816	LIST_HEAD(holdouts);
 817	unsigned long j;
 818	unsigned long lastinfo;
 819	unsigned long lastreport;
 820	bool reported = false;
 821	int rtsi;
 822	struct task_struct *t;
 823
 824	set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
 825	rtp->pregp_func(&holdouts);
 826
 827	/*
 828	 * There were callbacks, so we need to wait for an RCU-tasks
 829	 * grace period.  Start off by scanning the task list for tasks
 830	 * that are not already voluntarily blocked.  Mark these tasks
 831	 * and make a list of them in holdouts.
 832	 */
 833	set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
 834	if (rtp->pertask_func) {
 835		rcu_read_lock();
 836		for_each_process_thread(g, t)
 837			rtp->pertask_func(t, &holdouts);
 838		rcu_read_unlock();
 839	}
 840
 841	set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
 842	rtp->postscan_func(&holdouts);
 843
 844	/*
 845	 * Each pass through the following loop scans the list of holdout
 846	 * tasks, removing any that are no longer holdouts.  When the list
 847	 * is empty, we are done.
 848	 */
 849	lastreport = jiffies;
 850	lastinfo = lastreport;
 851	rtsi = READ_ONCE(rcu_task_stall_info);
 852
 853	// Start off with initial wait and slowly back off to 1 HZ wait.
 854	fract = rtp->init_fract;
 855
 856	while (!list_empty(&holdouts)) {
 857		ktime_t exp;
 858		bool firstreport;
 859		bool needreport;
 860		int rtst;
 861
 862		// Slowly back off waiting for holdouts
 
 
 
 863		set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
 864		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
 865			schedule_timeout_idle(fract);
 866		} else {
 867			exp = jiffies_to_nsecs(fract);
 868			__set_current_state(TASK_IDLE);
 869			schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
 870		}
 871
 872		if (fract < HZ)
 873			fract++;
 874
 875		rtst = READ_ONCE(rcu_task_stall_timeout);
 876		needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
 877		if (needreport) {
 878			lastreport = jiffies;
 879			reported = true;
 880		}
 881		firstreport = true;
 882		WARN_ON(signal_pending(current));
 883		set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
 884		rtp->holdouts_func(&holdouts, needreport, &firstreport);
 885
 886		// Print pre-stall informational messages if needed.
 887		j = jiffies;
 888		if (rtsi > 0 && !reported && time_after(j, lastinfo + rtsi)) {
 889			lastinfo = j;
 890			rtsi = rtsi * rcu_task_stall_info_mult;
 891			pr_info("%s: %s grace period number %lu (since boot) is %lu jiffies old.\n",
 892				__func__, rtp->kname, rtp->tasks_gp_seq, j - rtp->gp_start);
 893		}
 894	}
 895
 896	set_tasks_gp_state(rtp, RTGS_POST_GP);
 897	rtp->postgp_func(rtp);
 898}
 899
 900#endif /* #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU) */
 901
 902#ifdef CONFIG_TASKS_RCU
 903
 904////////////////////////////////////////////////////////////////////////
 905//
 906// Simple variant of RCU whose quiescent states are voluntary context
 907// switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle.
 908// As such, grace periods can take one good long time.  There are no
 909// read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
 910// because this implementation is intended to get the system into a safe
 911// state for some of the manipulations involved in tracing and the like.
 912// Finally, this implementation does not support high call_rcu_tasks()
 913// rates from multiple CPUs.  If this is required, per-CPU callback lists
 914// will be needed.
 915//
 916// The implementation uses rcu_tasks_wait_gp(), which relies on function
 917// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_kthread()
 918// function sets these function pointers up so that rcu_tasks_wait_gp()
 919// invokes these functions in this order:
 920//
 921// rcu_tasks_pregp_step():
 922//	Invokes synchronize_rcu() in order to wait for all in-flight
 923//	t->on_rq and t->nvcsw transitions to complete.	This works because
 924//	all such transitions are carried out with interrupts disabled.
 925// rcu_tasks_pertask(), invoked on every non-idle task:
 926//	For every runnable non-idle task other than the current one, use
 927//	get_task_struct() to pin down that task, snapshot that task's
 928//	number of voluntary context switches, and add that task to the
 929//	holdout list.
 930// rcu_tasks_postscan():
 931//	Gather per-CPU lists of tasks in do_exit() to ensure that all
 932//	tasks that were in the process of exiting (and which thus might
 933//	not know to synchronize with this RCU Tasks grace period) have
 934//	completed exiting.  The synchronize_rcu() in rcu_tasks_postgp()
 935//	will take care of any tasks stuck in the non-preemptible region
 936//	of do_exit() following its call to exit_tasks_rcu_finish().
 937// check_all_holdout_tasks(), repeatedly until holdout list is empty:
 938//	Scans the holdout list, attempting to identify a quiescent state
 939//	for each task on the list.  If there is a quiescent state, the
 940//	corresponding task is removed from the holdout list.
 941// rcu_tasks_postgp():
 942//	Invokes synchronize_rcu() in order to ensure that all prior
 943//	t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
 944//	to have happened before the end of this RCU Tasks grace period.
 945//	Again, this works because all such transitions are carried out
 946//	with interrupts disabled.
 947//
 948// For each exiting task, the exit_tasks_rcu_start() and
 949// exit_tasks_rcu_finish() functions add and remove, respectively, the
 950// current task to a per-CPU list of tasks that rcu_tasks_postscan() must
 951// wait on.  This is necessary because rcu_tasks_postscan() must wait on
 952// tasks that have already been removed from the global list of tasks.
 953//
 954// Pre-grace-period update-side code is ordered before the grace
 955// via the raw_spin_lock.*rcu_node().  Pre-grace-period read-side code
 956// is ordered before the grace period via synchronize_rcu() call in
 957// rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
 958// disabling.
 959
 960/* Pre-grace-period preparation. */
 961static void rcu_tasks_pregp_step(struct list_head *hop)
 962{
 963	/*
 964	 * Wait for all pre-existing t->on_rq and t->nvcsw transitions
 965	 * to complete.  Invoking synchronize_rcu() suffices because all
 966	 * these transitions occur with interrupts disabled.  Without this
 967	 * synchronize_rcu(), a read-side critical section that started
 968	 * before the grace period might be incorrectly seen as having
 969	 * started after the grace period.
 970	 *
 971	 * This synchronize_rcu() also dispenses with the need for a
 972	 * memory barrier on the first store to t->rcu_tasks_holdout,
 973	 * as it forces the store to happen after the beginning of the
 974	 * grace period.
 975	 */
 976	synchronize_rcu();
 977}
 978
 979/* Check for quiescent states since the pregp's synchronize_rcu() */
 980static bool rcu_tasks_is_holdout(struct task_struct *t)
 981{
 982	int cpu;
 983
 984	/* Has the task been seen voluntarily sleeping? */
 985	if (!READ_ONCE(t->on_rq))
 986		return false;
 987
 988	/*
 989	 * t->on_rq && !t->se.sched_delayed *could* be considered sleeping but
 990	 * since it is a spurious state (it will transition into the
 991	 * traditional blocked state or get woken up without outside
 992	 * dependencies), not considering it such should only affect timing.
 993	 *
 994	 * Be conservative for now and not include it.
 995	 */
 996
 997	/*
 998	 * Idle tasks (or idle injection) within the idle loop are RCU-tasks
 999	 * quiescent states. But CPU boot code performed by the idle task
1000	 * isn't a quiescent state.
1001	 */
1002	if (is_idle_task(t))
1003		return false;
1004
1005	cpu = task_cpu(t);
1006
1007	/* Idle tasks on offline CPUs are RCU-tasks quiescent states. */
1008	if (t == idle_task(cpu) && !rcu_cpu_online(cpu))
1009		return false;
1010
1011	return true;
1012}
1013
1014/* Per-task initial processing. */
1015static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
1016{
1017	if (t != current && rcu_tasks_is_holdout(t)) {
1018		get_task_struct(t);
1019		t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
1020		WRITE_ONCE(t->rcu_tasks_holdout, true);
1021		list_add(&t->rcu_tasks_holdout_list, hop);
1022	}
1023}
1024
1025void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
1026DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
1027
1028/* Processing between scanning taskslist and draining the holdout list. */
1029static void rcu_tasks_postscan(struct list_head *hop)
1030{
1031	int cpu;
1032	int rtsi = READ_ONCE(rcu_task_stall_info);
1033
1034	if (!IS_ENABLED(CONFIG_TINY_RCU)) {
1035		tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
1036		add_timer(&tasks_rcu_exit_srcu_stall_timer);
1037	}
1038
1039	/*
1040	 * Exiting tasks may escape the tasklist scan. Those are vulnerable
1041	 * until their final schedule() with TASK_DEAD state. To cope with
1042	 * this, divide the fragile exit path part in two intersecting
1043	 * read side critical sections:
1044	 *
1045	 * 1) A task_struct list addition before calling exit_notify(),
1046	 *    which may remove the task from the tasklist, with the
1047	 *    removal after the final preempt_disable() call in do_exit().
1048	 *
1049	 * 2) An _RCU_ read side starting with the final preempt_disable()
1050	 *    call in do_exit() and ending with the final call to schedule()
1051	 *    with TASK_DEAD state.
1052	 *
1053	 * This handles the part 1). And postgp will handle part 2) with a
1054	 * call to synchronize_rcu().
1055	 */
1056
1057	for_each_possible_cpu(cpu) {
1058		unsigned long j = jiffies + 1;
1059		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu);
1060		struct task_struct *t;
1061		struct task_struct *t1;
1062		struct list_head tmp;
1063
1064		raw_spin_lock_irq_rcu_node(rtpcp);
1065		list_for_each_entry_safe(t, t1, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) {
1066			if (list_empty(&t->rcu_tasks_holdout_list))
1067				rcu_tasks_pertask(t, hop);
1068
1069			// RT kernels need frequent pauses, otherwise
1070			// pause at least once per pair of jiffies.
1071			if (!IS_ENABLED(CONFIG_PREEMPT_RT) && time_before(jiffies, j))
1072				continue;
1073
1074			// Keep our place in the list while pausing.
1075			// Nothing else traverses this list, so adding a
1076			// bare list_head is OK.
1077			list_add(&tmp, &t->rcu_tasks_exit_list);
1078			raw_spin_unlock_irq_rcu_node(rtpcp);
1079			cond_resched(); // For CONFIG_PREEMPT=n kernels
1080			raw_spin_lock_irq_rcu_node(rtpcp);
1081			t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list);
1082			list_del(&tmp);
1083			j = jiffies + 1;
1084		}
1085		raw_spin_unlock_irq_rcu_node(rtpcp);
1086	}
1087
1088	if (!IS_ENABLED(CONFIG_TINY_RCU))
1089		del_timer_sync(&tasks_rcu_exit_srcu_stall_timer);
1090}
1091
1092/* See if tasks are still holding out, complain if so. */
1093static void check_holdout_task(struct task_struct *t,
1094			       bool needreport, bool *firstreport)
1095{
1096	int cpu;
1097
1098	if (!READ_ONCE(t->rcu_tasks_holdout) ||
1099	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
1100	    !rcu_tasks_is_holdout(t) ||
1101	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
1102	     !is_idle_task(t) && READ_ONCE(t->rcu_tasks_idle_cpu) >= 0)) {
1103		WRITE_ONCE(t->rcu_tasks_holdout, false);
1104		list_del_init(&t->rcu_tasks_holdout_list);
1105		put_task_struct(t);
1106		return;
1107	}
1108	rcu_request_urgent_qs_task(t);
1109	if (!needreport)
1110		return;
1111	if (*firstreport) {
1112		pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
1113		*firstreport = false;
1114	}
1115	cpu = task_cpu(t);
1116	pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
1117		 t, ".I"[is_idle_task(t)],
1118		 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
1119		 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
1120		 data_race(t->rcu_tasks_idle_cpu), cpu);
1121	sched_show_task(t);
1122}
1123
1124/* Scan the holdout lists for tasks no longer holding out. */
1125static void check_all_holdout_tasks(struct list_head *hop,
1126				    bool needreport, bool *firstreport)
1127{
1128	struct task_struct *t, *t1;
1129
1130	list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
1131		check_holdout_task(t, needreport, firstreport);
1132		cond_resched();
1133	}
1134}
1135
1136/* Finish off the Tasks-RCU grace period. */
1137static void rcu_tasks_postgp(struct rcu_tasks *rtp)
1138{
1139	/*
1140	 * Because ->on_rq and ->nvcsw are not guaranteed to have a full
1141	 * memory barriers prior to them in the schedule() path, memory
1142	 * reordering on other CPUs could cause their RCU-tasks read-side
1143	 * critical sections to extend past the end of the grace period.
1144	 * However, because these ->nvcsw updates are carried out with
1145	 * interrupts disabled, we can use synchronize_rcu() to force the
1146	 * needed ordering on all such CPUs.
1147	 *
1148	 * This synchronize_rcu() also confines all ->rcu_tasks_holdout
1149	 * accesses to be within the grace period, avoiding the need for
1150	 * memory barriers for ->rcu_tasks_holdout accesses.
1151	 *
1152	 * In addition, this synchronize_rcu() waits for exiting tasks
1153	 * to complete their final preempt_disable() region of execution,
1154	 * enforcing the whole region before tasklist removal until
1155	 * the final schedule() with TASK_DEAD state to be an RCU TASKS
1156	 * read side critical section.
1157	 */
1158	synchronize_rcu();
1159}
1160
1161static void tasks_rcu_exit_srcu_stall(struct timer_list *unused)
1162{
1163#ifndef CONFIG_TINY_RCU
1164	int rtsi;
1165
1166	rtsi = READ_ONCE(rcu_task_stall_info);
1167	pr_info("%s: %s grace period number %lu (since boot) gp_state: %s is %lu jiffies old.\n",
1168		__func__, rcu_tasks.kname, rcu_tasks.tasks_gp_seq,
1169		tasks_gp_state_getname(&rcu_tasks), jiffies - rcu_tasks.gp_jiffies);
1170	pr_info("Please check any exiting tasks stuck between calls to exit_tasks_rcu_start() and exit_tasks_rcu_finish()\n");
1171	tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
1172	add_timer(&tasks_rcu_exit_srcu_stall_timer);
1173#endif // #ifndef CONFIG_TINY_RCU
1174}
1175
1176/**
1177 * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
1178 * @rhp: structure to be used for queueing the RCU updates.
1179 * @func: actual callback function to be invoked after the grace period
1180 *
1181 * The callback function will be invoked some time after a full grace
1182 * period elapses, in other words after all currently executing RCU
1183 * read-side critical sections have completed. call_rcu_tasks() assumes
1184 * that the read-side critical sections end at a voluntary context
1185 * switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into idle,
1186 * or transition to usermode execution.  As such, there are no read-side
1187 * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
1188 * this primitive is intended to determine that all tasks have passed
1189 * through a safe state, not so much for data-structure synchronization.
1190 *
1191 * See the description of call_rcu() for more detailed information on
1192 * memory ordering guarantees.
1193 */
1194void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
1195{
1196	call_rcu_tasks_generic(rhp, func, &rcu_tasks);
1197}
1198EXPORT_SYMBOL_GPL(call_rcu_tasks);
1199
1200/**
1201 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
1202 *
1203 * Control will return to the caller some time after a full rcu-tasks
1204 * grace period has elapsed, in other words after all currently
1205 * executing rcu-tasks read-side critical sections have elapsed.  These
1206 * read-side critical sections are delimited by calls to schedule(),
1207 * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
1208 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
1209 *
1210 * This is a very specialized primitive, intended only for a few uses in
1211 * tracing and other situations requiring manipulation of function
1212 * preambles and profiling hooks.  The synchronize_rcu_tasks() function
1213 * is not (yet) intended for heavy use from multiple CPUs.
1214 *
1215 * See the description of synchronize_rcu() for more detailed information
1216 * on memory ordering guarantees.
1217 */
1218void synchronize_rcu_tasks(void)
1219{
1220	synchronize_rcu_tasks_generic(&rcu_tasks);
1221}
1222EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
1223
1224/**
1225 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
1226 *
1227 * Although the current implementation is guaranteed to wait, it is not
1228 * obligated to, for example, if there are no pending callbacks.
1229 */
1230void rcu_barrier_tasks(void)
1231{
1232	rcu_barrier_tasks_generic(&rcu_tasks);
 
1233}
1234EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
1235
1236static int rcu_tasks_lazy_ms = -1;
1237module_param(rcu_tasks_lazy_ms, int, 0444);
1238
1239static int __init rcu_spawn_tasks_kthread(void)
1240{
1241	rcu_tasks.gp_sleep = HZ / 10;
1242	rcu_tasks.init_fract = HZ / 10;
1243	if (rcu_tasks_lazy_ms >= 0)
1244		rcu_tasks.lazy_jiffies = msecs_to_jiffies(rcu_tasks_lazy_ms);
1245	rcu_tasks.pregp_func = rcu_tasks_pregp_step;
1246	rcu_tasks.pertask_func = rcu_tasks_pertask;
1247	rcu_tasks.postscan_func = rcu_tasks_postscan;
1248	rcu_tasks.holdouts_func = check_all_holdout_tasks;
1249	rcu_tasks.postgp_func = rcu_tasks_postgp;
1250	rcu_tasks.wait_state = TASK_IDLE;
1251	rcu_spawn_tasks_kthread_generic(&rcu_tasks);
1252	return 0;
1253}
 
1254
1255#if !defined(CONFIG_TINY_RCU)
1256void show_rcu_tasks_classic_gp_kthread(void)
1257{
1258	show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
1259}
1260EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
1261
1262void rcu_tasks_torture_stats_print(char *tt, char *tf)
1263{
1264	rcu_tasks_torture_stats_print_generic(&rcu_tasks, tt, tf, "");
1265}
1266EXPORT_SYMBOL_GPL(rcu_tasks_torture_stats_print);
1267#endif // !defined(CONFIG_TINY_RCU)
1268
1269struct task_struct *get_rcu_tasks_gp_kthread(void)
1270{
1271	return rcu_tasks.kthread_ptr;
1272}
1273EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
1274
1275void rcu_tasks_get_gp_data(int *flags, unsigned long *gp_seq)
1276{
1277	*flags = 0;
1278	*gp_seq = rcu_seq_current(&rcu_tasks.tasks_gp_seq);
1279}
1280EXPORT_SYMBOL_GPL(rcu_tasks_get_gp_data);
1281
1282/*
1283 * Protect against tasklist scan blind spot while the task is exiting and
1284 * may be removed from the tasklist.  Do this by adding the task to yet
1285 * another list.
1286 *
1287 * Note that the task will remove itself from this list, so there is no
1288 * need for get_task_struct(), except in the case where rcu_tasks_pertask()
1289 * adds it to the holdout list, in which case rcu_tasks_pertask() supplies
1290 * the needed get_task_struct().
1291 */
1292void exit_tasks_rcu_start(void)
1293{
1294	unsigned long flags;
1295	struct rcu_tasks_percpu *rtpcp;
1296	struct task_struct *t = current;
1297
1298	WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list));
1299	preempt_disable();
1300	rtpcp = this_cpu_ptr(rcu_tasks.rtpcpu);
1301	t->rcu_tasks_exit_cpu = smp_processor_id();
1302	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1303	WARN_ON_ONCE(!rtpcp->rtp_exit_list.next);
1304	list_add(&t->rcu_tasks_exit_list, &rtpcp->rtp_exit_list);
1305	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1306	preempt_enable();
1307}
1308
1309/*
1310 * Remove the task from the "yet another list" because do_exit() is now
1311 * non-preemptible, allowing synchronize_rcu() to wait beyond this point.
1312 */
1313void exit_tasks_rcu_finish(void)
1314{
1315	unsigned long flags;
1316	struct rcu_tasks_percpu *rtpcp;
1317	struct task_struct *t = current;
1318
1319	WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list));
1320	rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu);
1321	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1322	list_del_init(&t->rcu_tasks_exit_list);
1323	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1324
1325	exit_tasks_rcu_finish_trace(t);
1326}
1327
1328#else /* #ifdef CONFIG_TASKS_RCU */
 
1329void exit_tasks_rcu_start(void) { }
1330void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
1331#endif /* #else #ifdef CONFIG_TASKS_RCU */
1332
1333#ifdef CONFIG_TASKS_RUDE_RCU
1334
1335////////////////////////////////////////////////////////////////////////
1336//
1337// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's
1338// trick of passing an empty function to schedule_on_each_cpu().
1339// This approach provides batching of concurrent calls to the synchronous
1340// synchronize_rcu_tasks_rude() API.  This invokes schedule_on_each_cpu()
1341// in order to send IPIs far and wide and induces otherwise unnecessary
1342// context switches on all online CPUs, whether idle or not.
1343//
1344// Callback handling is provided by the rcu_tasks_kthread() function.
1345//
1346// Ordering is provided by the scheduler's context-switch code.
1347
1348// Empty function to allow workqueues to force a context switch.
1349static void rcu_tasks_be_rude(struct work_struct *work)
1350{
1351}
1352
1353// Wait for one rude RCU-tasks grace period.
1354static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
1355{
1356	rtp->n_ipis += cpumask_weight(cpu_online_mask);
1357	schedule_on_each_cpu(rcu_tasks_be_rude);
1358}
1359
1360static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
1361DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude,
1362		 "RCU Tasks Rude");
1363
1364/*
1365 * call_rcu_tasks_rude() - Queue a callback rude task-based grace period
1366 * @rhp: structure to be used for queueing the RCU updates.
1367 * @func: actual callback function to be invoked after the grace period
1368 *
1369 * The callback function will be invoked some time after a full grace
1370 * period elapses, in other words after all currently executing RCU
1371 * read-side critical sections have completed. call_rcu_tasks_rude()
1372 * assumes that the read-side critical sections end at context switch,
1373 * cond_resched_tasks_rcu_qs(), or transition to usermode execution (as
1374 * usermode execution is schedulable). As such, there are no read-side
1375 * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
1376 * this primitive is intended to determine that all tasks have passed
1377 * through a safe state, not so much for data-structure synchronization.
1378 *
1379 * See the description of call_rcu() for more detailed information on
1380 * memory ordering guarantees.
1381 *
1382 * This is no longer exported, and is instead reserved for use by
1383 * synchronize_rcu_tasks_rude().
1384 */
1385static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
1386{
1387	call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
1388}
 
1389
1390/**
1391 * synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
1392 *
1393 * Control will return to the caller some time after a rude rcu-tasks
1394 * grace period has elapsed, in other words after all currently
1395 * executing rcu-tasks read-side critical sections have elapsed.  These
1396 * read-side critical sections are delimited by calls to schedule(),
1397 * cond_resched_tasks_rcu_qs(), userspace execution (which is a schedulable
1398 * context), and (in theory, anyway) cond_resched().
1399 *
1400 * This is a very specialized primitive, intended only for a few uses in
1401 * tracing and other situations requiring manipulation of function preambles
1402 * and profiling hooks.  The synchronize_rcu_tasks_rude() function is not
1403 * (yet) intended for heavy use from multiple CPUs.
1404 *
1405 * See the description of synchronize_rcu() for more detailed information
1406 * on memory ordering guarantees.
1407 */
1408void synchronize_rcu_tasks_rude(void)
1409{
1410	if (!IS_ENABLED(CONFIG_ARCH_WANTS_NO_INSTR) || IS_ENABLED(CONFIG_FORCE_TASKS_RUDE_RCU))
1411		synchronize_rcu_tasks_generic(&rcu_tasks_rude);
1412}
1413EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
1414
 
 
 
 
 
 
 
 
 
 
 
 
 
1415static int __init rcu_spawn_tasks_rude_kthread(void)
1416{
1417	rcu_tasks_rude.gp_sleep = HZ / 10;
1418	rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
1419	return 0;
1420}
 
1421
1422#if !defined(CONFIG_TINY_RCU)
1423void show_rcu_tasks_rude_gp_kthread(void)
1424{
1425	show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
1426}
1427EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
1428
1429void rcu_tasks_rude_torture_stats_print(char *tt, char *tf)
1430{
1431	rcu_tasks_torture_stats_print_generic(&rcu_tasks_rude, tt, tf, "");
1432}
1433EXPORT_SYMBOL_GPL(rcu_tasks_rude_torture_stats_print);
1434#endif // !defined(CONFIG_TINY_RCU)
1435
1436struct task_struct *get_rcu_tasks_rude_gp_kthread(void)
1437{
1438	return rcu_tasks_rude.kthread_ptr;
1439}
1440EXPORT_SYMBOL_GPL(get_rcu_tasks_rude_gp_kthread);
1441
1442void rcu_tasks_rude_get_gp_data(int *flags, unsigned long *gp_seq)
1443{
1444	*flags = 0;
1445	*gp_seq = rcu_seq_current(&rcu_tasks_rude.tasks_gp_seq);
1446}
1447EXPORT_SYMBOL_GPL(rcu_tasks_rude_get_gp_data);
1448
1449#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
 
 
1450
1451////////////////////////////////////////////////////////////////////////
1452//
1453// Tracing variant of Tasks RCU.  This variant is designed to be used
1454// to protect tracing hooks, including those of BPF.  This variant
1455// therefore:
1456//
1457// 1.	Has explicit read-side markers to allow finite grace periods
1458//	in the face of in-kernel loops for PREEMPT=n builds.
1459//
1460// 2.	Protects code in the idle loop, exception entry/exit, and
1461//	CPU-hotplug code paths, similar to the capabilities of SRCU.
1462//
1463// 3.	Avoids expensive read-side instructions, having overhead similar
1464//	to that of Preemptible RCU.
1465//
1466// There are of course downsides.  For example, the grace-period code
1467// can send IPIs to CPUs, even when those CPUs are in the idle loop or
1468// in nohz_full userspace.  If needed, these downsides can be at least
1469// partially remedied.
 
1470//
1471// Perhaps most important, this variant of RCU does not affect the vanilla
1472// flavors, rcu_preempt and rcu_sched.  The fact that RCU Tasks Trace
1473// readers can operate from idle, offline, and exception entry/exit in no
1474// way allows rcu_preempt and rcu_sched readers to also do so.
1475//
1476// The implementation uses rcu_tasks_wait_gp(), which relies on function
1477// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_trace_kthread()
1478// function sets these function pointers up so that rcu_tasks_wait_gp()
1479// invokes these functions in this order:
1480//
1481// rcu_tasks_trace_pregp_step():
1482//	Disables CPU hotplug, adds all currently executing tasks to the
1483//	holdout list, then checks the state of all tasks that blocked
1484//	or were preempted within their current RCU Tasks Trace read-side
1485//	critical section, adding them to the holdout list if appropriate.
1486//	Finally, this function re-enables CPU hotplug.
1487// The ->pertask_func() pointer is NULL, so there is no per-task processing.
1488// rcu_tasks_trace_postscan():
1489//	Invokes synchronize_rcu() to wait for late-stage exiting tasks
1490//	to finish exiting.
1491// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
1492//	Scans the holdout list, attempting to identify a quiescent state
1493//	for each task on the list.  If there is a quiescent state, the
1494//	corresponding task is removed from the holdout list.  Once this
1495//	list is empty, the grace period has completed.
1496// rcu_tasks_trace_postgp():
1497//	Provides the needed full memory barrier and does debug checks.
1498//
1499// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
1500//
1501// Pre-grace-period update-side code is ordered before the grace period
1502// via the ->cbs_lock and barriers in rcu_tasks_kthread().  Pre-grace-period
1503// read-side code is ordered before the grace period by atomic operations
1504// on .b.need_qs flag of each task involved in this process, or by scheduler
1505// context-switch ordering (for locked-down non-running readers).
1506
1507// The lockdep state must be outside of #ifdef to be useful.
1508#ifdef CONFIG_DEBUG_LOCK_ALLOC
1509static struct lock_class_key rcu_lock_trace_key;
1510struct lockdep_map rcu_trace_lock_map =
1511	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_trace", &rcu_lock_trace_key);
1512EXPORT_SYMBOL_GPL(rcu_trace_lock_map);
1513#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
1514
1515#ifdef CONFIG_TASKS_TRACE_RCU
1516
 
 
 
1517// Record outstanding IPIs to each CPU.  No point in sending two...
1518static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
1519
1520// The number of detections of task quiescent state relying on
1521// heavyweight readers executing explicit memory barriers.
1522static unsigned long n_heavy_reader_attempts;
1523static unsigned long n_heavy_reader_updates;
1524static unsigned long n_heavy_reader_ofl_updates;
1525static unsigned long n_trc_holdouts;
1526
1527void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
1528DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
1529		 "RCU Tasks Trace");
1530
1531/* Load from ->trc_reader_special.b.need_qs with proper ordering. */
1532static u8 rcu_ld_need_qs(struct task_struct *t)
1533{
1534	smp_mb(); // Enforce full grace-period ordering.
1535	return smp_load_acquire(&t->trc_reader_special.b.need_qs);
1536}
1537
1538/* Store to ->trc_reader_special.b.need_qs with proper ordering. */
1539static void rcu_st_need_qs(struct task_struct *t, u8 v)
1540{
1541	smp_store_release(&t->trc_reader_special.b.need_qs, v);
1542	smp_mb(); // Enforce full grace-period ordering.
1543}
1544
1545/*
1546 * Do a cmpxchg() on ->trc_reader_special.b.need_qs, allowing for
1547 * the four-byte operand-size restriction of some platforms.
1548 *
1549 * Returns the old value, which is often ignored.
1550 */
1551u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
1552{
1553	return cmpxchg(&t->trc_reader_special.b.need_qs, old, new);
1554}
1555EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);
1556
1557/*
1558 * If we are the last reader, signal the grace-period kthread.
1559 * Also remove from the per-CPU list of blocked tasks.
1560 */
1561void rcu_read_unlock_trace_special(struct task_struct *t)
1562{
1563	unsigned long flags;
1564	struct rcu_tasks_percpu *rtpcp;
1565	union rcu_special trs;
1566
1567	// Open-coded full-word version of rcu_ld_need_qs().
1568	smp_mb(); // Enforce full grace-period ordering.
1569	trs = smp_load_acquire(&t->trc_reader_special);
1570
1571	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && t->trc_reader_special.b.need_mb)
1572		smp_mb(); // Pairs with update-side barriers.
1573	// Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
1574	if (trs.b.need_qs == (TRC_NEED_QS_CHECKED | TRC_NEED_QS)) {
1575		u8 result = rcu_trc_cmpxchg_need_qs(t, TRC_NEED_QS_CHECKED | TRC_NEED_QS,
1576						       TRC_NEED_QS_CHECKED);
1577
1578		WARN_ONCE(result != trs.b.need_qs, "%s: result = %d", __func__, result);
1579	}
1580	if (trs.b.blocked) {
1581		rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, t->trc_blkd_cpu);
1582		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1583		list_del_init(&t->trc_blkd_node);
1584		WRITE_ONCE(t->trc_reader_special.b.blocked, false);
1585		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1586	}
1587	WRITE_ONCE(t->trc_reader_nesting, 0);
1588}
1589EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
1590
1591/* Add a newly blocked reader task to its CPU's list. */
1592void rcu_tasks_trace_qs_blkd(struct task_struct *t)
1593{
1594	unsigned long flags;
1595	struct rcu_tasks_percpu *rtpcp;
1596
1597	local_irq_save(flags);
1598	rtpcp = this_cpu_ptr(rcu_tasks_trace.rtpcpu);
1599	raw_spin_lock_rcu_node(rtpcp); // irqs already disabled
1600	t->trc_blkd_cpu = smp_processor_id();
1601	if (!rtpcp->rtp_blkd_tasks.next)
1602		INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
1603	list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
1604	WRITE_ONCE(t->trc_reader_special.b.blocked, true);
1605	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1606}
1607EXPORT_SYMBOL_GPL(rcu_tasks_trace_qs_blkd);
1608
1609/* Add a task to the holdout list, if it is not already on the list. */
1610static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
1611{
1612	if (list_empty(&t->trc_holdout_list)) {
1613		get_task_struct(t);
1614		list_add(&t->trc_holdout_list, bhp);
1615		n_trc_holdouts++;
1616	}
1617}
1618
1619/* Remove a task from the holdout list, if it is in fact present. */
1620static void trc_del_holdout(struct task_struct *t)
1621{
1622	if (!list_empty(&t->trc_holdout_list)) {
1623		list_del_init(&t->trc_holdout_list);
1624		put_task_struct(t);
1625		n_trc_holdouts--;
1626	}
1627}
1628
1629/* IPI handler to check task state. */
1630static void trc_read_check_handler(void *t_in)
1631{
1632	int nesting;
1633	struct task_struct *t = current;
1634	struct task_struct *texp = t_in;
1635
1636	// If the task is no longer running on this CPU, leave.
1637	if (unlikely(texp != t))
 
 
1638		goto reset_ipi; // Already on holdout list, so will check later.
 
1639
1640	// If the task is not in a read-side critical section, and
1641	// if this is the last reader, awaken the grace-period kthread.
1642	nesting = READ_ONCE(t->trc_reader_nesting);
1643	if (likely(!nesting)) {
1644		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
 
 
 
1645		goto reset_ipi;
1646	}
1647	// If we are racing with an rcu_read_unlock_trace(), try again later.
1648	if (unlikely(nesting < 0))
1649		goto reset_ipi;
1650
1651	// Get here if the task is in a read-side critical section.
1652	// Set its state so that it will update state for the grace-period
1653	// kthread upon exit from that critical section.
1654	rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);
 
1655
1656reset_ipi:
1657	// Allow future IPIs to be sent on CPU and for task.
1658	// Also order this IPI handler against any later manipulations of
1659	// the intended task.
1660	smp_store_release(per_cpu_ptr(&trc_ipi_to_cpu, smp_processor_id()), false); // ^^^
1661	smp_store_release(&texp->trc_ipi_to_cpu, -1); // ^^^
1662}
1663
1664/* Callback function for scheduler to check locked-down task.  */
1665static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
1666{
1667	struct list_head *bhp = bhp_in;
1668	int cpu = task_cpu(t);
1669	int nesting;
1670	bool ofl = cpu_is_offline(cpu);
1671
1672	if (task_curr(t) && !ofl) {
 
 
1673		// If no chance of heavyweight readers, do it the hard way.
1674		if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
1675			return -EINVAL;
1676
1677		// If heavyweight readers are enabled on the remote task,
1678		// we can inspect its state despite its currently running.
1679		// However, we cannot safely change its state.
1680		n_heavy_reader_attempts++;
1681		// Check for "running" idle tasks on offline CPUs.
1682		if (!rcu_watching_zero_in_eqs(cpu, &t->trc_reader_nesting))
1683			return -EINVAL; // No quiescent state, do it the hard way.
1684		n_heavy_reader_updates++;
1685		nesting = 0;
1686	} else {
1687		// The task is not running, so C-language access is safe.
1688		nesting = t->trc_reader_nesting;
1689		WARN_ON_ONCE(ofl && task_curr(t) && (t != idle_task(task_cpu(t))));
1690		if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && ofl)
1691			n_heavy_reader_ofl_updates++;
 
 
 
1692	}
1693
1694	// If not exiting a read-side critical section, mark as checked
1695	// so that the grace-period kthread will remove it from the
1696	// holdout list.
1697	if (!nesting) {
1698		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1699		return 0;  // In QS, so done.
1700	}
1701	if (nesting < 0)
1702		return -EINVAL; // Reader transitioning, try again later.
1703
1704	// The task is in a read-side critical section, so set up its
1705	// state so that it will update state upon exit from that critical
1706	// section.
1707	if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
1708		trc_add_holdout(t, bhp);
1709	return 0;
 
1710}
1711
1712/* Attempt to extract the state for the specified task. */
1713static void trc_wait_for_one_reader(struct task_struct *t,
1714				    struct list_head *bhp)
1715{
1716	int cpu;
1717
1718	// If a previous IPI is still in flight, let it complete.
1719	if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
1720		return;
1721
1722	// The current task had better be in a quiescent state.
1723	if (t == current) {
1724		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1725		WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
 
1726		return;
1727	}
1728
1729	// Attempt to nail down the task for inspection.
1730	get_task_struct(t);
1731	if (!task_call_func(t, trc_inspect_reader, bhp)) {
1732		put_task_struct(t);
1733		return;
1734	}
1735	put_task_struct(t);
1736
1737	// If this task is not yet on the holdout list, then we are in
1738	// an RCU read-side critical section.  Otherwise, the invocation of
1739	// trc_add_holdout() that added it to the list did the necessary
1740	// get_task_struct().  Either way, the task cannot be freed out
1741	// from under this code.
1742
1743	// If currently running, send an IPI, either way, add to list.
1744	trc_add_holdout(t, bhp);
1745	if (task_curr(t) &&
1746	    time_after(jiffies + 1, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
1747		// The task is currently running, so try IPIing it.
1748		cpu = task_cpu(t);
1749
1750		// If there is already an IPI outstanding, let it happen.
1751		if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
1752			return;
1753
 
1754		per_cpu(trc_ipi_to_cpu, cpu) = true;
1755		t->trc_ipi_to_cpu = cpu;
1756		rcu_tasks_trace.n_ipis++;
1757		if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
 
1758			// Just in case there is some other reason for
1759			// failure than the target CPU being offline.
1760			WARN_ONCE(1, "%s():  smp_call_function_single() failed for CPU: %d\n",
1761				  __func__, cpu);
1762			rcu_tasks_trace.n_ipis_fails++;
1763			per_cpu(trc_ipi_to_cpu, cpu) = false;
1764			t->trc_ipi_to_cpu = -1;
 
 
 
 
1765		}
1766	}
1767}
1768
1769/*
1770 * Initialize for first-round processing for the specified task.
1771 * Return false if task is NULL or already taken care of, true otherwise.
1772 */
1773static bool rcu_tasks_trace_pertask_prep(struct task_struct *t, bool notself)
1774{
1775	// During early boot when there is only the one boot CPU, there
1776	// is no idle task for the other CPUs.	Also, the grace-period
1777	// kthread is always in a quiescent state.  In addition, just return
1778	// if this task is already on the list.
1779	if (unlikely(t == NULL) || (t == current && notself) || !list_empty(&t->trc_holdout_list))
1780		return false;
1781
1782	rcu_st_need_qs(t, 0);
1783	t->trc_ipi_to_cpu = -1;
1784	return true;
1785}
1786
1787/* Do first-round processing for the specified task. */
1788static void rcu_tasks_trace_pertask(struct task_struct *t, struct list_head *hop)
1789{
1790	if (rcu_tasks_trace_pertask_prep(t, true))
1791		trc_wait_for_one_reader(t, hop);
1792}
1793
1794/* Initialize for a new RCU-tasks-trace grace period. */
1795static void rcu_tasks_trace_pregp_step(struct list_head *hop)
1796{
1797	LIST_HEAD(blkd_tasks);
1798	int cpu;
1799	unsigned long flags;
1800	struct rcu_tasks_percpu *rtpcp;
1801	struct task_struct *t;
1802
1803	// There shouldn't be any old IPIs, but...
1804	for_each_possible_cpu(cpu)
1805		WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
1806
1807	// Disable CPU hotplug across the CPU scan for the benefit of
1808	// any IPIs that might be needed.  This also waits for all readers
1809	// in CPU-hotplug code paths.
1810	cpus_read_lock();
 
1811
1812	// These rcu_tasks_trace_pertask_prep() calls are serialized to
1813	// allow safe access to the hop list.
1814	for_each_online_cpu(cpu) {
1815		rcu_read_lock();
1816		// Note that cpu_curr_snapshot() picks up the target
1817		// CPU's current task while its runqueue is locked with
1818		// an smp_mb__after_spinlock().  This ensures that either
1819		// the grace-period kthread will see that task's read-side
1820		// critical section or the task will see the updater's pre-GP
1821		// accesses.  The trailing smp_mb() in cpu_curr_snapshot()
1822		// does not currently play a role other than simplify
1823		// that function's ordering semantics.  If these simplified
1824		// ordering semantics continue to be redundant, that smp_mb()
1825		// might be removed.
1826		t = cpu_curr_snapshot(cpu);
1827		if (rcu_tasks_trace_pertask_prep(t, true))
1828			trc_add_holdout(t, hop);
1829		rcu_read_unlock();
1830		cond_resched_tasks_rcu_qs();
1831	}
1832
1833	// Only after all running tasks have been accounted for is it
1834	// safe to take care of the tasks that have blocked within their
1835	// current RCU tasks trace read-side critical section.
1836	for_each_possible_cpu(cpu) {
1837		rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, cpu);
1838		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1839		list_splice_init(&rtpcp->rtp_blkd_tasks, &blkd_tasks);
1840		while (!list_empty(&blkd_tasks)) {
1841			rcu_read_lock();
1842			t = list_first_entry(&blkd_tasks, struct task_struct, trc_blkd_node);
1843			list_del_init(&t->trc_blkd_node);
1844			list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
1845			raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1846			rcu_tasks_trace_pertask(t, hop);
1847			rcu_read_unlock();
1848			raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1849		}
1850		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1851		cond_resched_tasks_rcu_qs();
1852	}
1853
1854	// Re-enable CPU hotplug now that the holdout list is populated.
1855	cpus_read_unlock();
1856}
1857
1858/*
1859 * Do intermediate processing between task and holdout scans.
 
1860 */
1861static void rcu_tasks_trace_postscan(struct list_head *hop)
1862{
1863	// Wait for late-stage exiting tasks to finish exiting.
1864	// These might have passed the call to exit_tasks_rcu_finish().
1865
1866	// If you remove the following line, update rcu_trace_implies_rcu_gp()!!!
1867	synchronize_rcu();
1868	// Any tasks that exit after this point will set
1869	// TRC_NEED_QS_CHECKED in ->trc_reader_special.b.need_qs.
1870}
1871
1872/* Communicate task state back to the RCU tasks trace stall warning request. */
1873struct trc_stall_chk_rdr {
1874	int nesting;
1875	int ipi_to_cpu;
1876	u8 needqs;
1877};
1878
1879static int trc_check_slow_task(struct task_struct *t, void *arg)
1880{
1881	struct trc_stall_chk_rdr *trc_rdrp = arg;
1882
1883	if (task_curr(t) && cpu_online(task_cpu(t)))
1884		return false; // It is running, so decline to inspect it.
1885	trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
1886	trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
1887	trc_rdrp->needqs = rcu_ld_need_qs(t);
1888	return true;
1889}
1890
1891/* Show the state of a task stalling the current RCU tasks trace GP. */
1892static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
1893{
1894	int cpu;
1895	struct trc_stall_chk_rdr trc_rdr;
1896	bool is_idle_tsk = is_idle_task(t);
1897
1898	if (*firstreport) {
1899		pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
1900		*firstreport = false;
1901	}
 
1902	cpu = task_cpu(t);
1903	if (!task_call_func(t, trc_check_slow_task, &trc_rdr))
1904		pr_alert("P%d: %c%c\n",
1905			 t->pid,
1906			 ".I"[t->trc_ipi_to_cpu >= 0],
1907			 ".i"[is_idle_tsk]);
1908	else
1909		pr_alert("P%d: %c%c%c%c nesting: %d%c%c cpu: %d%s\n",
1910			 t->pid,
1911			 ".I"[trc_rdr.ipi_to_cpu >= 0],
1912			 ".i"[is_idle_tsk],
1913			 ".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
1914			 ".B"[!!data_race(t->trc_reader_special.b.blocked)],
1915			 trc_rdr.nesting,
1916			 " !CN"[trc_rdr.needqs & 0x3],
1917			 " ?"[trc_rdr.needqs > 0x3],
1918			 cpu, cpu_online(cpu) ? "" : "(offline)");
1919	sched_show_task(t);
1920}
1921
1922/* List stalled IPIs for RCU tasks trace. */
1923static void show_stalled_ipi_trace(void)
1924{
1925	int cpu;
1926
1927	for_each_possible_cpu(cpu)
1928		if (per_cpu(trc_ipi_to_cpu, cpu))
1929			pr_alert("\tIPI outstanding to CPU %d\n", cpu);
1930}
1931
1932/* Do one scan of the holdout list. */
1933static void check_all_holdout_tasks_trace(struct list_head *hop,
1934					  bool needreport, bool *firstreport)
1935{
1936	struct task_struct *g, *t;
1937
1938	// Disable CPU hotplug across the holdout list scan for IPIs.
1939	cpus_read_lock();
1940
1941	list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
1942		// If safe and needed, try to check the current task.
1943		if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
1944		    !(rcu_ld_need_qs(t) & TRC_NEED_QS_CHECKED))
1945			trc_wait_for_one_reader(t, hop);
1946
1947		// If check succeeded, remove this task from the list.
1948		if (smp_load_acquire(&t->trc_ipi_to_cpu) == -1 &&
1949		    rcu_ld_need_qs(t) == TRC_NEED_QS_CHECKED)
1950			trc_del_holdout(t);
1951		else if (needreport)
1952			show_stalled_task_trace(t, firstreport);
1953		cond_resched_tasks_rcu_qs();
1954	}
1955
1956	// Re-enable CPU hotplug now that the holdout list scan has completed.
1957	cpus_read_unlock();
1958
1959	if (needreport) {
1960		if (*firstreport)
1961			pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
1962		show_stalled_ipi_trace();
1963	}
1964}
1965
1966static void rcu_tasks_trace_empty_fn(void *unused)
1967{
1968}
1969
1970/* Wait for grace period to complete and provide ordering. */
1971static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
1972{
1973	int cpu;
 
 
 
1974
1975	// Wait for any lingering IPI handlers to complete.  Note that
1976	// if a CPU has gone offline or transitioned to userspace in the
1977	// meantime, all IPI handlers should have been drained beforehand.
1978	// Yes, this assumes that CPUs process IPIs in order.  If that ever
1979	// changes, there will need to be a recheck and/or timed wait.
1980	for_each_online_cpu(cpu)
1981		if (WARN_ON_ONCE(smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))))
1982			smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);
1983
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1984	smp_mb(); // Caller's code must be ordered after wakeup.
1985		  // Pairs with pretty much every ordering primitive.
1986}
1987
1988/* Report any needed quiescent state for this exiting task. */
1989static void exit_tasks_rcu_finish_trace(struct task_struct *t)
1990{
1991	union rcu_special trs = READ_ONCE(t->trc_reader_special);
1992
1993	rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1994	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
1995	if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS || trs.b.blocked))
1996		rcu_read_unlock_trace_special(t);
1997	else
1998		WRITE_ONCE(t->trc_reader_nesting, 0);
1999}
2000
2001/**
2002 * call_rcu_tasks_trace() - Queue a callback trace task-based grace period
2003 * @rhp: structure to be used for queueing the RCU updates.
2004 * @func: actual callback function to be invoked after the grace period
2005 *
2006 * The callback function will be invoked some time after a trace rcu-tasks
2007 * grace period elapses, in other words after all currently executing
2008 * trace rcu-tasks read-side critical sections have completed. These
2009 * read-side critical sections are delimited by calls to rcu_read_lock_trace()
2010 * and rcu_read_unlock_trace().
 
 
 
 
2011 *
2012 * See the description of call_rcu() for more detailed information on
2013 * memory ordering guarantees.
2014 */
2015void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func)
2016{
2017	call_rcu_tasks_generic(rhp, func, &rcu_tasks_trace);
2018}
2019EXPORT_SYMBOL_GPL(call_rcu_tasks_trace);
2020
2021/**
2022 * synchronize_rcu_tasks_trace - wait for a trace rcu-tasks grace period
2023 *
2024 * Control will return to the caller some time after a trace rcu-tasks
2025 * grace period has elapsed, in other words after all currently executing
2026 * trace rcu-tasks read-side critical sections have elapsed. These read-side
2027 * critical sections are delimited by calls to rcu_read_lock_trace()
2028 * and rcu_read_unlock_trace().
2029 *
2030 * This is a very specialized primitive, intended only for a few uses in
2031 * tracing and other situations requiring manipulation of function preambles
2032 * and profiling hooks.  The synchronize_rcu_tasks_trace() function is not
2033 * (yet) intended for heavy use from multiple CPUs.
2034 *
2035 * See the description of synchronize_rcu() for more detailed information
2036 * on memory ordering guarantees.
2037 */
2038void synchronize_rcu_tasks_trace(void)
2039{
2040	RCU_LOCKDEP_WARN(lock_is_held(&rcu_trace_lock_map), "Illegal synchronize_rcu_tasks_trace() in RCU Tasks Trace read-side critical section");
2041	synchronize_rcu_tasks_generic(&rcu_tasks_trace);
2042}
2043EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_trace);
2044
2045/**
2046 * rcu_barrier_tasks_trace - Wait for in-flight call_rcu_tasks_trace() callbacks.
2047 *
2048 * Although the current implementation is guaranteed to wait, it is not
2049 * obligated to, for example, if there are no pending callbacks.
2050 */
2051void rcu_barrier_tasks_trace(void)
2052{
2053	rcu_barrier_tasks_generic(&rcu_tasks_trace);
 
2054}
2055EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);
2056
2057int rcu_tasks_trace_lazy_ms = -1;
2058module_param(rcu_tasks_trace_lazy_ms, int, 0444);
2059
2060static int __init rcu_spawn_tasks_trace_kthread(void)
2061{
2062	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
2063		rcu_tasks_trace.gp_sleep = HZ / 10;
2064		rcu_tasks_trace.init_fract = HZ / 10;
2065	} else {
2066		rcu_tasks_trace.gp_sleep = HZ / 200;
2067		if (rcu_tasks_trace.gp_sleep <= 0)
2068			rcu_tasks_trace.gp_sleep = 1;
2069		rcu_tasks_trace.init_fract = HZ / 200;
2070		if (rcu_tasks_trace.init_fract <= 0)
2071			rcu_tasks_trace.init_fract = 1;
2072	}
2073	if (rcu_tasks_trace_lazy_ms >= 0)
2074		rcu_tasks_trace.lazy_jiffies = msecs_to_jiffies(rcu_tasks_trace_lazy_ms);
2075	rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
 
2076	rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
2077	rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
2078	rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
2079	rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
2080	return 0;
2081}
 
2082
2083#if !defined(CONFIG_TINY_RCU)
2084void show_rcu_tasks_trace_gp_kthread(void)
2085{
2086	char buf[64];
2087
2088	snprintf(buf, sizeof(buf), "N%lu h:%lu/%lu/%lu",
2089		data_race(n_trc_holdouts),
2090		data_race(n_heavy_reader_ofl_updates),
2091		data_race(n_heavy_reader_updates),
2092		data_race(n_heavy_reader_attempts));
2093	show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
2094}
2095EXPORT_SYMBOL_GPL(show_rcu_tasks_trace_gp_kthread);
2096
2097void rcu_tasks_trace_torture_stats_print(char *tt, char *tf)
2098{
2099	rcu_tasks_torture_stats_print_generic(&rcu_tasks_trace, tt, tf, "");
2100}
2101EXPORT_SYMBOL_GPL(rcu_tasks_trace_torture_stats_print);
2102#endif // !defined(CONFIG_TINY_RCU)
2103
2104struct task_struct *get_rcu_tasks_trace_gp_kthread(void)
2105{
2106	return rcu_tasks_trace.kthread_ptr;
2107}
2108EXPORT_SYMBOL_GPL(get_rcu_tasks_trace_gp_kthread);
2109
2110void rcu_tasks_trace_get_gp_data(int *flags, unsigned long *gp_seq)
2111{
2112	*flags = 0;
2113	*gp_seq = rcu_seq_current(&rcu_tasks_trace.tasks_gp_seq);
2114}
2115EXPORT_SYMBOL_GPL(rcu_tasks_trace_get_gp_data);
2116
2117#else /* #ifdef CONFIG_TASKS_TRACE_RCU */
2118static void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
 
2119#endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */
2120
2121#ifndef CONFIG_TINY_RCU
2122void show_rcu_tasks_gp_kthreads(void)
2123{
2124	show_rcu_tasks_classic_gp_kthread();
2125	show_rcu_tasks_rude_gp_kthread();
2126	show_rcu_tasks_trace_gp_kthread();
2127}
2128#endif /* #ifndef CONFIG_TINY_RCU */
2129
2130#ifdef CONFIG_PROVE_RCU
2131struct rcu_tasks_test_desc {
2132	struct rcu_head rh;
2133	const char *name;
2134	bool notrun;
2135	unsigned long runstart;
2136};
2137
2138static struct rcu_tasks_test_desc tests[] = {
2139	{
2140		.name = "call_rcu_tasks()",
2141		/* If not defined, the test is skipped. */
2142		.notrun = IS_ENABLED(CONFIG_TASKS_RCU),
2143	},
2144	{
2145		.name = "call_rcu_tasks_trace()",
2146		/* If not defined, the test is skipped. */
2147		.notrun = IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
2148	}
2149};
2150
2151#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
2152static void test_rcu_tasks_callback(struct rcu_head *rhp)
2153{
2154	struct rcu_tasks_test_desc *rttd =
2155		container_of(rhp, struct rcu_tasks_test_desc, rh);
2156
2157	pr_info("Callback from %s invoked.\n", rttd->name);
2158
2159	rttd->notrun = false;
2160}
2161#endif // #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
2162
2163static void rcu_tasks_initiate_self_tests(void)
2164{
2165#ifdef CONFIG_TASKS_RCU
2166	pr_info("Running RCU Tasks wait API self tests\n");
2167	tests[0].runstart = jiffies;
2168	synchronize_rcu_tasks();
2169	call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
2170#endif
2171
2172#ifdef CONFIG_TASKS_RUDE_RCU
2173	pr_info("Running RCU Tasks Rude wait API self tests\n");
2174	synchronize_rcu_tasks_rude();
2175#endif
2176
2177#ifdef CONFIG_TASKS_TRACE_RCU
2178	pr_info("Running RCU Tasks Trace wait API self tests\n");
2179	tests[1].runstart = jiffies;
2180	synchronize_rcu_tasks_trace();
2181	call_rcu_tasks_trace(&tests[1].rh, test_rcu_tasks_callback);
2182#endif
2183}
2184
2185/*
2186 * Return:  0 - test passed
2187 *	    1 - test failed, but have not timed out yet
2188 *	   -1 - test failed and timed out
2189 */
2190static int rcu_tasks_verify_self_tests(void)
2191{
2192	int ret = 0;
2193	int i;
2194	unsigned long bst = rcu_task_stall_timeout;
2195
2196	if (bst <= 0 || bst > RCU_TASK_BOOT_STALL_TIMEOUT)
2197		bst = RCU_TASK_BOOT_STALL_TIMEOUT;
2198	for (i = 0; i < ARRAY_SIZE(tests); i++) {
2199		while (tests[i].notrun) {		// still hanging.
2200			if (time_after(jiffies, tests[i].runstart + bst)) {
2201				pr_err("%s has failed boot-time tests.\n", tests[i].name);
2202				ret = -1;
2203				break;
2204			}
2205			ret = 1;
2206			break;
2207		}
2208	}
2209	WARN_ON(ret < 0);
2210
2211	return ret;
2212}
2213
2214/*
2215 * Repeat the rcu_tasks_verify_self_tests() call once every second until the
2216 * test passes or has timed out.
2217 */
2218static struct delayed_work rcu_tasks_verify_work;
2219static void rcu_tasks_verify_work_fn(struct work_struct *work __maybe_unused)
2220{
2221	int ret = rcu_tasks_verify_self_tests();
2222
2223	if (ret <= 0)
2224		return;
2225
2226	/* Test fails but not timed out yet, reschedule another check */
2227	schedule_delayed_work(&rcu_tasks_verify_work, HZ);
2228}
2229
2230static int rcu_tasks_verify_schedule_work(void)
2231{
2232	INIT_DELAYED_WORK(&rcu_tasks_verify_work, rcu_tasks_verify_work_fn);
2233	rcu_tasks_verify_work_fn(NULL);
2234	return 0;
2235}
2236late_initcall(rcu_tasks_verify_schedule_work);
2237#else /* #ifdef CONFIG_PROVE_RCU */
2238static void rcu_tasks_initiate_self_tests(void) { }
2239#endif /* #else #ifdef CONFIG_PROVE_RCU */
2240
2241void __init tasks_cblist_init_generic(void)
2242{
2243	lockdep_assert_irqs_disabled();
2244	WARN_ON(num_online_cpus() > 1);
2245
2246#ifdef CONFIG_TASKS_RCU
2247	cblist_init_generic(&rcu_tasks);
2248#endif
2249
2250#ifdef CONFIG_TASKS_RUDE_RCU
2251	cblist_init_generic(&rcu_tasks_rude);
2252#endif
2253
2254#ifdef CONFIG_TASKS_TRACE_RCU
2255	cblist_init_generic(&rcu_tasks_trace);
2256#endif
2257}
2258
2259void __init rcu_init_tasks_generic(void)
2260{
2261#ifdef CONFIG_TASKS_RCU
2262	rcu_spawn_tasks_kthread();
2263#endif
2264
2265#ifdef CONFIG_TASKS_RUDE_RCU
2266	rcu_spawn_tasks_rude_kthread();
2267#endif
2268
2269#ifdef CONFIG_TASKS_TRACE_RCU
2270	rcu_spawn_tasks_trace_kthread();
2271#endif
2272
2273	// Run the self-tests.
2274	rcu_tasks_initiate_self_tests();
2275}
2276
2277#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
2278static inline void rcu_tasks_bootup_oddness(void) {}
 
2279#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */