1#include <linux/atomic.h>
  2#include <linux/rwsem.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/errno.h>
 10
 11int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
 12			const char *name, struct lock_class_key *rwsem_key)
 13{
 14	brw->fast_read_ctr = alloc_percpu(int);
 15	if (unlikely(!brw->fast_read_ctr))
 16		return -ENOMEM;
 17
 18	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
 19	__init_rwsem(&brw->rw_sem, name, rwsem_key);
 20	rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
 21	atomic_set(&brw->slow_read_ctr, 0);
 22	init_waitqueue_head(&brw->write_waitq);
 23	return 0;
 24}
 25EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
 26
 27void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
 28{
 29	/*
 30	 * XXX: temporary kludge. The error path in alloc_super()
 31	 * assumes that percpu_free_rwsem() is safe after kzalloc().
 32	 */
 33	if (!brw->fast_read_ctr)
 34		return;
 35
 36	rcu_sync_dtor(&brw->rss);
 37	free_percpu(brw->fast_read_ctr);
 38	brw->fast_read_ctr = NULL; /* catch use after free bugs */
 39}
 40
 41/*
 42 * This is the fast-path for down_read/up_read. If it succeeds we rely
 43 * on the barriers provided by rcu_sync_enter/exit; see the comments in
 44 * percpu_down_write() and percpu_up_write().
 45 *
 46 * If this helper fails the callers rely on the normal rw_semaphore and
 47 * atomic_dec_and_test(), so in this case we have the necessary barriers.
 48 */
 49static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
 50{
 51	bool success;
 52
 53	preempt_disable();
 54	success = rcu_sync_is_idle(&brw->rss);
 55	if (likely(success))
 56		__this_cpu_add(*brw->fast_read_ctr, val);
 57	preempt_enable();
 58
 59	return success;
 60}
 61
 62/*
 63 * Like the normal down_read() this is not recursive, the writer can
 64 * come after the first percpu_down_read() and create the deadlock.
 65 *
 66 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
 67 * percpu_up_read() does rwsem_release(). This pairs with the usage
 68 * of ->rw_sem in percpu_down/up_write().
 69 */
 70void percpu_down_read(struct percpu_rw_semaphore *brw)
 71{
 72	might_sleep();
 73	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
 74
 75	if (likely(update_fast_ctr(brw, +1)))
 76		return;
 77
 78	/* Avoid rwsem_acquire_read() and rwsem_release() */
 79	__down_read(&brw->rw_sem);
 80	atomic_inc(&brw->slow_read_ctr);
 81	__up_read(&brw->rw_sem);
 82}
 83EXPORT_SYMBOL_GPL(percpu_down_read);
 84
 85int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
 86{
 87	if (unlikely(!update_fast_ctr(brw, +1))) {
 88		if (!__down_read_trylock(&brw->rw_sem))
 89			return 0;
 90		atomic_inc(&brw->slow_read_ctr);
 91		__up_read(&brw->rw_sem);
 92	}
 93
 94	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
 95	return 1;
 96}
 97
 98void percpu_up_read(struct percpu_rw_semaphore *brw)
 99{
100	rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
101
102	if (likely(update_fast_ctr(brw, -1)))
103		return;
104
105	/* false-positive is possible but harmless */
106	if (atomic_dec_and_test(&brw->slow_read_ctr))
107		wake_up_all(&brw->write_waitq);
108}
109EXPORT_SYMBOL_GPL(percpu_up_read);
110
111static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
112{
113	unsigned int sum = 0;
114	int cpu;
115
116	for_each_possible_cpu(cpu) {
117		sum += per_cpu(*brw->fast_read_ctr, cpu);
118		per_cpu(*brw->fast_read_ctr, cpu) = 0;
119	}
120
121	return sum;
122}
123
124void percpu_down_write(struct percpu_rw_semaphore *brw)
125{
126	/*
127	 * Make rcu_sync_is_idle() == F and thus disable the fast-path in
128	 * percpu_down_read() and percpu_up_read(), and wait for gp pass.
129	 *
130	 * The latter synchronises us with the preceding readers which used
131	 * the fast-past, so we can not miss the result of __this_cpu_add()
132	 * or anything else inside their criticial sections.
133	 */
134	rcu_sync_enter(&brw->rss);
135
136	/* exclude other writers, and block the new readers completely */
137	down_write(&brw->rw_sem);
138
139	/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
140	atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
141
142	/* wait for all readers to complete their percpu_up_read() */
143	wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
144}
145EXPORT_SYMBOL_GPL(percpu_down_write);
146
147void percpu_up_write(struct percpu_rw_semaphore *brw)
148{
149	/* release the lock, but the readers can't use the fast-path */
150	up_write(&brw->rw_sem);
151	/*
152	 * Enable the fast-path in percpu_down_read() and percpu_up_read()
153	 * but only after another gp pass; this adds the necessary barrier
154	 * to ensure the reader can't miss the changes done by us.
155	 */
156	rcu_sync_exit(&brw->rss);
157}
158EXPORT_SYMBOL_GPL(percpu_up_write);