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