srcutiny.c

  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/irq_work.h>
 13#include <linux/mutex.h>
 14#include <linux/preempt.h>
 15#include <linux/rcupdate_wait.h>
 16#include <linux/sched.h>
 17#include <linux/delay.h>
 18#include <linux/srcu.h>
 19
 20#include <linux/rcu_node_tree.h>
 21#include "rcu_segcblist.h"
 22#include "rcu.h"
 23
 24#ifndef CONFIG_TREE_RCU
 25int rcu_scheduler_active __read_mostly;
 26#else // #ifndef CONFIG_TREE_RCU
 27extern int rcu_scheduler_active;
 28#endif // #else // #ifndef CONFIG_TREE_RCU
 29static LIST_HEAD(srcu_boot_list);
 30static bool srcu_init_done;
 31
 32static int init_srcu_struct_fields(struct srcu_struct *ssp)
 33{
 34	ssp->srcu_lock_nesting[0] = 0;
 35	ssp->srcu_lock_nesting[1] = 0;
 36	init_swait_queue_head(&ssp->srcu_wq);
 37	ssp->srcu_cb_head = NULL;
 38	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
 39	ssp->srcu_gp_running = false;
 40	ssp->srcu_gp_waiting = false;
 41	ssp->srcu_idx = 0;
 42	ssp->srcu_idx_max = 0;
 43	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
 44	INIT_LIST_HEAD(&ssp->srcu_work.entry);
 45	init_irq_work(&ssp->srcu_irq_work, srcu_tiny_irq_work);
 46	return 0;
 47}
 48
 49#ifdef CONFIG_DEBUG_LOCK_ALLOC
 50
 51int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
 52		       struct lock_class_key *key)
 53{
 54	/* Don't re-initialize a lock while it is held. */
 55	debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
 56	lockdep_init_map(&ssp->dep_map, name, key, 0);
 57	return init_srcu_struct_fields(ssp);
 58}
 59EXPORT_SYMBOL_GPL(__init_srcu_struct);
 60
 61#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 62
 63/*
 64 * init_srcu_struct - initialize a sleep-RCU structure
 65 * @ssp: structure to initialize.
 66 *
 67 * Must invoke this on a given srcu_struct before passing that srcu_struct
 68 * to any other function.  Each srcu_struct represents a separate domain
 69 * of SRCU protection.
 70 */
 71int init_srcu_struct(struct srcu_struct *ssp)
 72{
 73	return init_srcu_struct_fields(ssp);
 74}
 75EXPORT_SYMBOL_GPL(init_srcu_struct);
 76
 77#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 78
 79/*
 80 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
 81 * @ssp: structure to clean up.
 82 *
 83 * Must invoke this after you are finished using a given srcu_struct that
 84 * was initialized via init_srcu_struct(), else you leak memory.
 85 */
 86void cleanup_srcu_struct(struct srcu_struct *ssp)
 87{
 88	WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
 89	irq_work_sync(&ssp->srcu_irq_work);
 90	flush_work(&ssp->srcu_work);
 91	WARN_ON(ssp->srcu_gp_running);
 92	WARN_ON(ssp->srcu_gp_waiting);
 93	WARN_ON(ssp->srcu_cb_head);
 94	WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
 95	WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
 96	WARN_ON(ssp->srcu_idx & 0x1);
 97}
 98EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 99
100/*
101 * Removes the count for the old reader from the appropriate element of
102 * the srcu_struct.
103 */
104void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
105{
106	int newval;
107
108	preempt_disable();  // Needed for PREEMPT_LAZY
109	newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1;
110	WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
111	preempt_enable();
112	if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task() && !irqs_disabled())
113		swake_up_one(&ssp->srcu_wq);
114}
115EXPORT_SYMBOL_GPL(__srcu_read_unlock);
116
117/*
118 * Workqueue handler to drive one grace period and invoke any callbacks
119 * that become ready as a result.  Single-CPU operation and preemption
120 * disabling mean that we get away with murder on synchronization.  ;-)
121 */
122void srcu_drive_gp(struct work_struct *wp)
123{
124	int idx;
125	struct rcu_head *lh;
126	struct rcu_head *rhp;
127	struct srcu_struct *ssp;
128
129	ssp = container_of(wp, struct srcu_struct, srcu_work);
130	preempt_disable();  // Needed for PREEMPT_LAZY
131	if (ssp->srcu_gp_running || ULONG_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) {
132		preempt_enable();
133		return; /* Already running or nothing to do. */
134	}
135
136	/* Remove recently arrived callbacks and wait for readers. */
137	WRITE_ONCE(ssp->srcu_gp_running, true);
138	local_irq_disable();
139	lh = ssp->srcu_cb_head;
140	ssp->srcu_cb_head = NULL;
141	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
142	local_irq_enable();
143	idx = (ssp->srcu_idx & 0x2) / 2;
144	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
145	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
146	preempt_enable();
147	do {
148		// Deadlock issues prevent __srcu_read_unlock() from
149		// doing an unconditional wakeup, so polling is required.
150		swait_event_timeout_exclusive(ssp->srcu_wq,
151					      !READ_ONCE(ssp->srcu_lock_nesting[idx]), HZ / 10);
152	} while (READ_ONCE(ssp->srcu_lock_nesting[idx]));
153	preempt_disable();  // Needed for PREEMPT_LAZY
154	WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
155	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
156	preempt_enable();
157
158	/* Invoke the callbacks we removed above. */
159	while (lh) {
160		rhp = lh;
161		lh = lh->next;
162		debug_rcu_head_callback(rhp);
163		local_bh_disable();
164		rhp->func(rhp);
165		local_bh_enable();
166	}
167
168	/*
169	 * Enable rescheduling, and if there are more callbacks,
170	 * reschedule ourselves.  This can race with a call_srcu()
171	 * at interrupt level, but the ->srcu_gp_running checks will
172	 * straighten that out.
173	 */
174	preempt_disable();  // Needed for PREEMPT_LAZY
175	WRITE_ONCE(ssp->srcu_gp_running, false);
176	idx = ULONG_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max));
177	preempt_enable();
178	if (idx)
179		schedule_work(&ssp->srcu_work);
180}
181EXPORT_SYMBOL_GPL(srcu_drive_gp);
182
183/*
184 * Use an irq_work to defer schedule_work() to avoid acquiring the workqueue
185 * pool->lock while the caller might hold scheduler locks, causing lockdep
186 * splats due to workqueue_init() doing a wakeup.
187 */
188void srcu_tiny_irq_work(struct irq_work *irq_work)
189{
190	struct srcu_struct *ssp;
191
192	ssp = container_of(irq_work, struct srcu_struct, srcu_irq_work);
193	schedule_work(&ssp->srcu_work);
194}
195EXPORT_SYMBOL_GPL(srcu_tiny_irq_work);
196
197static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
198{
199	unsigned long cookie;
200
201	lockdep_assert_preemption_disabled(); // Needed for PREEMPT_LAZY
202	cookie = get_state_synchronize_srcu(ssp);
203	if (ULONG_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) {
204		return;
205	}
206	WRITE_ONCE(ssp->srcu_idx_max, cookie);
207	if (!READ_ONCE(ssp->srcu_gp_running)) {
208		if (likely(srcu_init_done))
209			irq_work_queue(&ssp->srcu_irq_work);
210		else if (list_empty(&ssp->srcu_work.entry))
211			list_add(&ssp->srcu_work.entry, &srcu_boot_list);
212	}
213}
214
215/*
216 * Enqueue an SRCU callback on the specified srcu_struct structure,
217 * initiating grace-period processing if it is not already running.
218 */
219void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
220	       rcu_callback_t func)
221{
222	unsigned long flags;
223
224	rhp->func = func;
225	rhp->next = NULL;
226	preempt_disable();  // Needed for PREEMPT_LAZY
227	local_irq_save(flags);
228	*ssp->srcu_cb_tail = rhp;
229	ssp->srcu_cb_tail = &rhp->next;
230	local_irq_restore(flags);
231	srcu_gp_start_if_needed(ssp);
232	preempt_enable();
233}
234EXPORT_SYMBOL_GPL(call_srcu);
235
236/*
237 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
238 */
239void synchronize_srcu(struct srcu_struct *ssp)
240{
241	struct rcu_synchronize rs;
242
243	srcu_lock_sync(&ssp->dep_map);
244
245	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
246			lock_is_held(&rcu_bh_lock_map) ||
247			lock_is_held(&rcu_lock_map) ||
248			lock_is_held(&rcu_sched_lock_map),
249			"Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
250
251	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
252		return;
253
254	might_sleep();
255	init_rcu_head_on_stack(&rs.head);
256	init_completion(&rs.completion);
257	call_srcu(ssp, &rs.head, wakeme_after_rcu);
258	wait_for_completion(&rs.completion);
259	destroy_rcu_head_on_stack(&rs.head);
260}
261EXPORT_SYMBOL_GPL(synchronize_srcu);
262
263/*
264 * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
265 */
266unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
267{
268	unsigned long ret;
269
270	barrier();
271	ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
272	barrier();
273	return ret;
274}
275EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
276
277/*
278 * start_poll_synchronize_srcu - Provide cookie and start grace period
279 *
280 * The difference between this and get_state_synchronize_srcu() is that
281 * this function ensures that the poll_state_synchronize_srcu() will
282 * eventually return the value true.
283 */
284unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
285{
286	unsigned long ret;
287
288	preempt_disable();  // Needed for PREEMPT_LAZY
289	ret = get_state_synchronize_srcu(ssp);
290	srcu_gp_start_if_needed(ssp);
291	preempt_enable();
292	return ret;
293}
294EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
295
296/*
297 * poll_state_synchronize_srcu - Has cookie's grace period ended?
298 */
299bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
300{
301	unsigned long cur_s = READ_ONCE(ssp->srcu_idx);
302
303	barrier();
304	return cookie == SRCU_GET_STATE_COMPLETED ||
305	       ULONG_CMP_GE(cur_s, cookie) || ULONG_CMP_LT(cur_s, cookie - 3);
306}
307EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
308
309#ifndef CONFIG_TREE_RCU
310/* Lockdep diagnostics.  */
311void __init rcu_scheduler_starting(void)
312{
313	rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
314}
315#endif // #ifndef CONFIG_TREE_RCU
316
317/*
318 * Queue work for srcu_struct structures with early boot callbacks.
319 * The work won't actually execute until the workqueue initialization
320 * phase that takes place after the scheduler starts.
321 */
322void __init srcu_init(void)
323{
324	struct srcu_struct *ssp;
325
326	srcu_init_done = true;
327	while (!list_empty(&srcu_boot_list)) {
328		ssp = list_first_entry(&srcu_boot_list,
329				      struct srcu_struct, srcu_work.entry);
330		list_del_init(&ssp->srcu_work.entry);
331		schedule_work(&ssp->srcu_work);
332	}
333}