1// SPDX-License-Identifier: GPL-2.0-only
  2#include <linux/atomic.h>
  3#include <linux/rwsem.h>
  4#include <linux/percpu.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/errno.h>
 10
 11#include "rwsem.h"
 12
 13int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
 14			const char *name, struct lock_class_key *rwsem_key)
 15{
 16	sem->read_count = alloc_percpu(int);
 17	if (unlikely(!sem->read_count))
 18		return -ENOMEM;
 19
 20	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
 21	rcu_sync_init(&sem->rss);
 22	__init_rwsem(&sem->rw_sem, name, rwsem_key);
 23	rcuwait_init(&sem->writer);
 24	sem->readers_block = 0;
 25	return 0;
 26}
 27EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
 28
 29void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
 30{
 31	/*
 32	 * XXX: temporary kludge. The error path in alloc_super()
 33	 * assumes that percpu_free_rwsem() is safe after kzalloc().
 34	 */
 35	if (!sem->read_count)
 36		return;
 37
 38	rcu_sync_dtor(&sem->rss);
 39	free_percpu(sem->read_count);
 40	sem->read_count = NULL; /* catch use after free bugs */
 41}
 42EXPORT_SYMBOL_GPL(percpu_free_rwsem);
 43
 44int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
 45{
 46	/*
 47	 * Due to having preemption disabled the decrement happens on
 48	 * the same CPU as the increment, avoiding the
 49	 * increment-on-one-CPU-and-decrement-on-another problem.
 50	 *
 51	 * If the reader misses the writer's assignment of readers_block, then
 52	 * the writer is guaranteed to see the reader's increment.
 53	 *
 54	 * Conversely, any readers that increment their sem->read_count after
 55	 * the writer looks are guaranteed to see the readers_block value,
 56	 * which in turn means that they are guaranteed to immediately
 57	 * decrement their sem->read_count, so that it doesn't matter that the
 58	 * writer missed them.
 59	 */
 60
 61	smp_mb(); /* A matches D */
 62
 63	/*
 64	 * If !readers_block the critical section starts here, matched by the
 65	 * release in percpu_up_write().
 66	 */
 67	if (likely(!smp_load_acquire(&sem->readers_block)))
 68		return 1;
 69
 70	/*
 71	 * Per the above comment; we still have preemption disabled and
 72	 * will thus decrement on the same CPU as we incremented.
 73	 */
 74	__percpu_up_read(sem);
 75
 76	if (try)
 77		return 0;
 78
 79	/*
 80	 * We either call schedule() in the wait, or we'll fall through
 81	 * and reschedule on the preempt_enable() in percpu_down_read().
 82	 */
 83	preempt_enable_no_resched();
 84
 85	/*
 86	 * Avoid lockdep for the down/up_read() we already have them.
 87	 */
 88	__down_read(&sem->rw_sem);
 89	this_cpu_inc(*sem->read_count);
 90	__up_read(&sem->rw_sem);
 91
 92	preempt_disable();
 93	return 1;
 94}
 95EXPORT_SYMBOL_GPL(__percpu_down_read);
 96
 97void __percpu_up_read(struct percpu_rw_semaphore *sem)
 98{
 99	smp_mb(); /* B matches C */
100	/*
101	 * In other words, if they see our decrement (presumably to aggregate
102	 * zero, as that is the only time it matters) they will also see our
103	 * critical section.
104	 */
105	__this_cpu_dec(*sem->read_count);
106
107	/* Prod writer to recheck readers_active */
108	rcuwait_wake_up(&sem->writer);
109}
110EXPORT_SYMBOL_GPL(__percpu_up_read);
111
112#define per_cpu_sum(var)						\
113({									\
114	typeof(var) __sum = 0;						\
115	int cpu;							\
116	compiletime_assert_atomic_type(__sum);				\
117	for_each_possible_cpu(cpu)					\
118		__sum += per_cpu(var, cpu);				\
119	__sum;								\
120})
121
122/*
123 * Return true if the modular sum of the sem->read_count per-CPU variable is
124 * zero.  If this sum is zero, then it is stable due to the fact that if any
125 * newly arriving readers increment a given counter, they will immediately
126 * decrement that same counter.
127 */
128static bool readers_active_check(struct percpu_rw_semaphore *sem)
129{
130	if (per_cpu_sum(*sem->read_count) != 0)
131		return false;
132
133	/*
134	 * If we observed the decrement; ensure we see the entire critical
135	 * section.
136	 */
137
138	smp_mb(); /* C matches B */
139
140	return true;
141}
142
143void percpu_down_write(struct percpu_rw_semaphore *sem)
144{
145	/* Notify readers to take the slow path. */
146	rcu_sync_enter(&sem->rss);
147
148	down_write(&sem->rw_sem);
149
150	/*
151	 * Notify new readers to block; up until now, and thus throughout the
152	 * longish rcu_sync_enter() above, new readers could still come in.
153	 */
154	WRITE_ONCE(sem->readers_block, 1);
155
156	smp_mb(); /* D matches A */
157
158	/*
159	 * If they don't see our writer of readers_block, then we are
160	 * guaranteed to see their sem->read_count increment, and therefore
161	 * will wait for them.
162	 */
163
164	/* Wait for all now active readers to complete. */
165	rcuwait_wait_event(&sem->writer, readers_active_check(sem));
166}
167EXPORT_SYMBOL_GPL(percpu_down_write);
168
169void percpu_up_write(struct percpu_rw_semaphore *sem)
170{
171	/*
172	 * Signal the writer is done, no fast path yet.
173	 *
174	 * One reason that we cannot just immediately flip to readers_fast is
175	 * that new readers might fail to see the results of this writer's
176	 * critical section.
177	 *
178	 * Therefore we force it through the slow path which guarantees an
179	 * acquire and thereby guarantees the critical section's consistency.
180	 */
181	smp_store_release(&sem->readers_block, 0);
182
183	/*
184	 * Release the write lock, this will allow readers back in the game.
185	 */
186	up_write(&sem->rw_sem);
187
188	/*
189	 * Once this completes (at least one RCU-sched grace period hence) the
190	 * reader fast path will be available again. Safe to use outside the
191	 * exclusive write lock because its counting.
192	 */
193	rcu_sync_exit(&sem->rss);
194}
195EXPORT_SYMBOL_GPL(percpu_up_write);