1// SPDX-License-Identifier: GPL-2.0+
  2/*
  3 * Sleepable Read-Copy Update mechanism for mutual exclusion,
  4 *	tiny version for non-preemptible single-CPU use.
  5 *
  6 * Copyright (C) IBM Corporation, 2017
  7 *
  8 * Author: Paul McKenney <paulmck@linux.ibm.com>
  9 */
 10
 11#include <linux/export.h>
 12#include <linux/mutex.h>
 13#include <linux/preempt.h>
 14#include <linux/rcupdate_wait.h>
 15#include <linux/sched.h>
 16#include <linux/delay.h>
 17#include <linux/srcu.h>
 18
 19#include <linux/rcu_node_tree.h>
 20#include "rcu_segcblist.h"
 21#include "rcu.h"
 22
 23int rcu_scheduler_active __read_mostly;
 24static LIST_HEAD(srcu_boot_list);
 25static bool srcu_init_done;
 26
 27static int init_srcu_struct_fields(struct srcu_struct *ssp)
 28{
 29	ssp->srcu_lock_nesting[0] = 0;
 30	ssp->srcu_lock_nesting[1] = 0;
 31	init_swait_queue_head(&ssp->srcu_wq);
 32	ssp->srcu_cb_head = NULL;
 33	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
 34	ssp->srcu_gp_running = false;
 35	ssp->srcu_gp_waiting = false;
 36	ssp->srcu_idx = 0;
 37	ssp->srcu_idx_max = 0;
 38	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
 39	INIT_LIST_HEAD(&ssp->srcu_work.entry);
 40	return 0;
 41}
 42
 43#ifdef CONFIG_DEBUG_LOCK_ALLOC
 44
 45int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
 46		       struct lock_class_key *key)
 47{
 48	/* Don't re-initialize a lock while it is held. */
 49	debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
 50	lockdep_init_map(&ssp->dep_map, name, key, 0);
 51	return init_srcu_struct_fields(ssp);
 52}
 53EXPORT_SYMBOL_GPL(__init_srcu_struct);
 54
 55#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 56
 57/*
 58 * init_srcu_struct - initialize a sleep-RCU structure
 59 * @ssp: structure to initialize.
 60 *
 61 * Must invoke this on a given srcu_struct before passing that srcu_struct
 62 * to any other function.  Each srcu_struct represents a separate domain
 63 * of SRCU protection.
 64 */
 65int init_srcu_struct(struct srcu_struct *ssp)
 66{
 67	return init_srcu_struct_fields(ssp);
 68}
 69EXPORT_SYMBOL_GPL(init_srcu_struct);
 70
 71#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 72
 73/*
 74 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
 75 * @ssp: structure to clean up.
 76 *
 77 * Must invoke this after you are finished using a given srcu_struct that
 78 * was initialized via init_srcu_struct(), else you leak memory.
 79 */
 80void cleanup_srcu_struct(struct srcu_struct *ssp)
 81{
 82	WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
 83	flush_work(&ssp->srcu_work);
 84	WARN_ON(ssp->srcu_gp_running);
 85	WARN_ON(ssp->srcu_gp_waiting);
 86	WARN_ON(ssp->srcu_cb_head);
 87	WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
 88	WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
 89	WARN_ON(ssp->srcu_idx & 0x1);
 90}
 91EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 92
 93/*
 94 * Removes the count for the old reader from the appropriate element of
 95 * the srcu_struct.
 96 */
 97void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
 98{
 99	int newval = ssp->srcu_lock_nesting[idx] - 1;
100
101	WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
102	if (!newval && READ_ONCE(ssp->srcu_gp_waiting))
103		swake_up_one(&ssp->srcu_wq);
104}
105EXPORT_SYMBOL_GPL(__srcu_read_unlock);
106
107/*
108 * Workqueue handler to drive one grace period and invoke any callbacks
109 * that become ready as a result.  Single-CPU and !PREEMPTION operation
110 * means that we get away with murder on synchronization.  ;-)
111 */
112void srcu_drive_gp(struct work_struct *wp)
113{
114	int idx;
115	struct rcu_head *lh;
116	struct rcu_head *rhp;
117	struct srcu_struct *ssp;
118
119	ssp = container_of(wp, struct srcu_struct, srcu_work);
120	if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
121		return; /* Already running or nothing to do. */
122
123	/* Remove recently arrived callbacks and wait for readers. */
124	WRITE_ONCE(ssp->srcu_gp_running, true);
125	local_irq_disable();
126	lh = ssp->srcu_cb_head;
127	ssp->srcu_cb_head = NULL;
128	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
129	local_irq_enable();
130	idx = (ssp->srcu_idx & 0x2) / 2;
131	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
132	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
133	swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
134	WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
135	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
136
137	/* Invoke the callbacks we removed above. */
138	while (lh) {
139		rhp = lh;
140		lh = lh->next;
141		local_bh_disable();
142		rhp->func(rhp);
143		local_bh_enable();
144	}
145
146	/*
147	 * Enable rescheduling, and if there are more callbacks,
148	 * reschedule ourselves.  This can race with a call_srcu()
149	 * at interrupt level, but the ->srcu_gp_running checks will
150	 * straighten that out.
151	 */
152	WRITE_ONCE(ssp->srcu_gp_running, false);
153	if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
154		schedule_work(&ssp->srcu_work);
155}
156EXPORT_SYMBOL_GPL(srcu_drive_gp);
157
158static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
159{
160	unsigned short cookie;
161
162	cookie = get_state_synchronize_srcu(ssp);
163	if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
164		return;
165	WRITE_ONCE(ssp->srcu_idx_max, cookie);
166	if (!READ_ONCE(ssp->srcu_gp_running)) {
167		if (likely(srcu_init_done))
168			schedule_work(&ssp->srcu_work);
169		else if (list_empty(&ssp->srcu_work.entry))
170			list_add(&ssp->srcu_work.entry, &srcu_boot_list);
171	}
172}
173
174/*
175 * Enqueue an SRCU callback on the specified srcu_struct structure,
176 * initiating grace-period processing if it is not already running.
177 */
178void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
179	       rcu_callback_t func)
180{
181	unsigned long flags;
182
183	rhp->func = func;
184	rhp->next = NULL;
185	local_irq_save(flags);
186	*ssp->srcu_cb_tail = rhp;
187	ssp->srcu_cb_tail = &rhp->next;
188	local_irq_restore(flags);
189	srcu_gp_start_if_needed(ssp);
190}
191EXPORT_SYMBOL_GPL(call_srcu);
192
193/*
194 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
195 */
196void synchronize_srcu(struct srcu_struct *ssp)
197{
198	struct rcu_synchronize rs;
199
200	init_rcu_head_on_stack(&rs.head);
201	init_completion(&rs.completion);
202	call_srcu(ssp, &rs.head, wakeme_after_rcu);
203	wait_for_completion(&rs.completion);
204	destroy_rcu_head_on_stack(&rs.head);
205}
206EXPORT_SYMBOL_GPL(synchronize_srcu);
207
208/*
209 * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
210 */
211unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
212{
213	unsigned long ret;
214
215	barrier();
216	ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
217	barrier();
218	return ret & USHRT_MAX;
219}
220EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
221
222/*
223 * start_poll_synchronize_srcu - Provide cookie and start grace period
224 *
225 * The difference between this and get_state_synchronize_srcu() is that
226 * this function ensures that the poll_state_synchronize_srcu() will
227 * eventually return the value true.
228 */
229unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
230{
231	unsigned long ret = get_state_synchronize_srcu(ssp);
232
233	srcu_gp_start_if_needed(ssp);
234	return ret;
235}
236EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
237
238/*
239 * poll_state_synchronize_srcu - Has cookie's grace period ended?
240 */
241bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
242{
243	bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
244
245	barrier();
246	return ret;
247}
248EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
249
250/* Lockdep diagnostics.  */
251void __init rcu_scheduler_starting(void)
252{
253	rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
254}
255
256/*
257 * Queue work for srcu_struct structures with early boot callbacks.
258 * The work won't actually execute until the workqueue initialization
259 * phase that takes place after the scheduler starts.
260 */
261void __init srcu_init(void)
262{
263	struct srcu_struct *ssp;
264
265	srcu_init_done = true;
266	while (!list_empty(&srcu_boot_list)) {
267		ssp = list_first_entry(&srcu_boot_list,
268				      struct srcu_struct, srcu_work.entry);
269		list_del_init(&ssp->srcu_work.entry);
270		schedule_work(&ssp->srcu_work);
271	}
272}