percpu-rwsem.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2#include <linux/atomic.h>
  3#include <linux/percpu.h>
  4#include <linux/wait.h>
  5#include <linux/lockdep.h>
  6#include <linux/percpu-rwsem.h>
  7#include <linux/rcupdate.h>
  8#include <linux/sched.h>
  9#include <linux/sched/task.h>
 10#include <linux/sched/debug.h>
 11#include <linux/errno.h>
 12#include <trace/events/lock.h>
 13
 14int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
 15			const char *name, struct lock_class_key *key)
 16{
 17	sem->read_count = alloc_percpu(int);
 18	if (unlikely(!sem->read_count))
 19		return -ENOMEM;
 20
 21	rcu_sync_init(&sem->rss);
 22	rcuwait_init(&sem->writer);
 23	init_waitqueue_head(&sem->waiters);
 24	atomic_set(&sem->block, 0);
 25#ifdef CONFIG_DEBUG_LOCK_ALLOC
 26	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
 27	lockdep_init_map(&sem->dep_map, name, key, 0);
 28#endif
 29	return 0;
 30}
 31EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
 32
 33void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
 34{
 35	/*
 36	 * XXX: temporary kludge. The error path in alloc_super()
 37	 * assumes that percpu_free_rwsem() is safe after kzalloc().
 38	 */
 39	if (!sem->read_count)
 40		return;
 41
 42	rcu_sync_dtor(&sem->rss);
 43	free_percpu(sem->read_count);
 44	sem->read_count = NULL; /* catch use after free bugs */
 45}
 46EXPORT_SYMBOL_GPL(percpu_free_rwsem);
 47
 48static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 49{
 50	this_cpu_inc(*sem->read_count);
 51
 52	/*
 53	 * Due to having preemption disabled the decrement happens on
 54	 * the same CPU as the increment, avoiding the
 55	 * increment-on-one-CPU-and-decrement-on-another problem.
 56	 *
 57	 * If the reader misses the writer's assignment of sem->block, then the
 58	 * writer is guaranteed to see the reader's increment.
 59	 *
 60	 * Conversely, any readers that increment their sem->read_count after
 61	 * the writer looks are guaranteed to see the sem->block value, which
 62	 * in turn means that they are guaranteed to immediately decrement
 63	 * their sem->read_count, so that it doesn't matter that the writer
 64	 * missed them.
 65	 */
 66
 67	smp_mb(); /* A matches D */
 68
 69	/*
 70	 * If !sem->block the critical section starts here, matched by the
 71	 * release in percpu_up_write().
 72	 */
 73	if (likely(!atomic_read_acquire(&sem->block)))
 74		return true;
 75
 76	this_cpu_dec(*sem->read_count);
 77
 78	/* Prod writer to re-evaluate readers_active_check() */
 79	rcuwait_wake_up(&sem->writer);
 80
 81	return false;
 82}
 83
 84static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
 85{
 86	if (atomic_read(&sem->block))
 87		return false;
 88
 89	return atomic_xchg(&sem->block, 1) == 0;
 90}
 91
 92static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
 93{
 94	if (reader) {
 95		bool ret;
 96
 97		preempt_disable();
 98		ret = __percpu_down_read_trylock(sem);
 99		preempt_enable();
100
101		return ret;
102	}
103	return __percpu_down_write_trylock(sem);
104}
105
106/*
107 * The return value of wait_queue_entry::func means:
108 *
109 *  <0 - error, wakeup is terminated and the error is returned
110 *   0 - no wakeup, a next waiter is tried
111 *  >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
112 *
113 * We use EXCLUSIVE for both readers and writers to preserve FIFO order,
114 * and play games with the return value to allow waking multiple readers.
115 *
116 * Specifically, we wake readers until we've woken a single writer, or until a
117 * trylock fails.
118 */
119static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
120				      unsigned int mode, int wake_flags,
121				      void *key)
122{
123	bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
124	struct percpu_rw_semaphore *sem = key;
125	struct task_struct *p;
126
127	/* concurrent against percpu_down_write(), can get stolen */
128	if (!__percpu_rwsem_trylock(sem, reader))
129		return 1;
130
131	p = get_task_struct(wq_entry->private);
132	list_del_init(&wq_entry->entry);
133	smp_store_release(&wq_entry->private, NULL);
134
135	wake_up_process(p);
136	put_task_struct(p);
137
138	return !reader; /* wake (readers until) 1 writer */
139}
140
141static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader,
142			      bool freeze)
143{
144	DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
145	bool wait;
146
147	spin_lock_irq(&sem->waiters.lock);
148	/*
149	 * Serialize against the wakeup in percpu_up_write(), if we fail
150	 * the trylock, the wakeup must see us on the list.
151	 */
152	wait = !__percpu_rwsem_trylock(sem, reader);
153	if (wait) {
154		wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
155		__add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
156	}
157	spin_unlock_irq(&sem->waiters.lock);
158
159	while (wait) {
160		set_current_state(TASK_UNINTERRUPTIBLE |
161				  (freeze ? TASK_FREEZABLE : 0));
162		if (!smp_load_acquire(&wq_entry.private))
163			break;
164		schedule();
165	}
166	__set_current_state(TASK_RUNNING);
167}
168
169bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try,
170				bool freeze)
171{
172	if (__percpu_down_read_trylock(sem))
173		return true;
174
175	if (try)
176		return false;
177
178	trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
179	preempt_enable();
180	percpu_rwsem_wait(sem, /* .reader = */ true, freeze);
181	preempt_disable();
182	trace_contention_end(sem, 0);
183
184	return true;
185}
186EXPORT_SYMBOL_GPL(__percpu_down_read);
187
188#define per_cpu_sum(var)						\
189({									\
190	TYPEOF_UNQUAL(var) __sum = 0;					\
191	int cpu;							\
192	compiletime_assert_atomic_type(__sum);				\
193	for_each_possible_cpu(cpu)					\
194		__sum += per_cpu(var, cpu);				\
195	__sum;								\
196})
197
198bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
199{
200	return per_cpu_sum(*sem->read_count) != 0 && !atomic_read(&sem->block);
201}
202EXPORT_SYMBOL_GPL(percpu_is_read_locked);
203
204/*
205 * Return true if the modular sum of the sem->read_count per-CPU variable is
206 * zero.  If this sum is zero, then it is stable due to the fact that if any
207 * newly arriving readers increment a given counter, they will immediately
208 * decrement that same counter.
209 *
210 * Assumes sem->block is set.
211 */
212static bool readers_active_check(struct percpu_rw_semaphore *sem)
213{
214	if (per_cpu_sum(*sem->read_count) != 0)
215		return false;
216
217	/*
218	 * If we observed the decrement; ensure we see the entire critical
219	 * section.
220	 */
221
222	smp_mb(); /* C matches B */
223
224	return true;
225}
226
227void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
228{
229	bool contended = false;
230
231	might_sleep();
232	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
233
234	/* Notify readers to take the slow path. */
235	rcu_sync_enter(&sem->rss);
236
237	/*
238	 * Try set sem->block; this provides writer-writer exclusion.
239	 * Having sem->block set makes new readers block.
240	 */
241	if (!__percpu_down_write_trylock(sem)) {
242		trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
243		percpu_rwsem_wait(sem, /* .reader = */ false, false);
244		contended = true;
245	}
246
247	/* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
248
249	/*
250	 * If they don't see our store of sem->block, then we are guaranteed to
251	 * see their sem->read_count increment, and therefore will wait for
252	 * them.
253	 */
254
255	/* Wait for all active readers to complete. */
256	rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
257	if (contended)
258		trace_contention_end(sem, 0);
259}
260EXPORT_SYMBOL_GPL(percpu_down_write);
261
262void percpu_up_write(struct percpu_rw_semaphore *sem)
263{
264	rwsem_release(&sem->dep_map, _RET_IP_);
265
266	/*
267	 * Signal the writer is done, no fast path yet.
268	 *
269	 * One reason that we cannot just immediately flip to readers_fast is
270	 * that new readers might fail to see the results of this writer's
271	 * critical section.
272	 *
273	 * Therefore we force it through the slow path which guarantees an
274	 * acquire and thereby guarantees the critical section's consistency.
275	 */
276	atomic_set_release(&sem->block, 0);
277
278	/*
279	 * Prod any pending reader/writer to make progress.
280	 */
281	__wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
282
283	/*
284	 * Once this completes (at least one RCU-sched grace period hence) the
285	 * reader fast path will be available again. Safe to use outside the
286	 * exclusive write lock because its counting.
287	 */
288	rcu_sync_exit(&sem->rss);
289}
290EXPORT_SYMBOL_GPL(percpu_up_write);