160 lines
4.4 KiB
C
160 lines
4.4 KiB
C
#include <linux/atomic.h>
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#include <linux/rwsem.h>
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#include <linux/percpu.h>
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#include <linux/wait.h>
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#include <linux/lockdep.h>
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#include <linux/percpu-rwsem.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/errno.h>
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int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
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const char *name, struct lock_class_key *rwsem_key)
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{
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brw->fast_read_ctr = alloc_percpu(int);
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if (unlikely(!brw->fast_read_ctr))
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return -ENOMEM;
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/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
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__init_rwsem(&brw->rw_sem, name, rwsem_key);
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rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
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atomic_set(&brw->slow_read_ctr, 0);
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init_waitqueue_head(&brw->write_waitq);
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return 0;
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}
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EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
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void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
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{
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/*
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* XXX: temporary kludge. The error path in alloc_super()
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* assumes that percpu_free_rwsem() is safe after kzalloc().
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*/
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if (!brw->fast_read_ctr)
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return;
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rcu_sync_dtor(&brw->rss);
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free_percpu(brw->fast_read_ctr);
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brw->fast_read_ctr = NULL; /* catch use after free bugs */
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}
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EXPORT_SYMBOL_GPL(percpu_free_rwsem);
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/*
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* This is the fast-path for down_read/up_read. If it succeeds we rely
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* on the barriers provided by rcu_sync_enter/exit; see the comments in
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* percpu_down_write() and percpu_up_write().
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*
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* If this helper fails the callers rely on the normal rw_semaphore and
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* atomic_dec_and_test(), so in this case we have the necessary barriers.
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*/
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static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
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{
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bool success;
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preempt_disable();
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success = rcu_sync_is_idle(&brw->rss);
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if (likely(success))
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__this_cpu_add(*brw->fast_read_ctr, val);
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preempt_enable();
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return success;
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}
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/*
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* Like the normal down_read() this is not recursive, the writer can
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* come after the first percpu_down_read() and create the deadlock.
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*
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* Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
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* percpu_up_read() does rwsem_release(). This pairs with the usage
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* of ->rw_sem in percpu_down/up_write().
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*/
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void percpu_down_read(struct percpu_rw_semaphore *brw)
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{
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might_sleep();
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rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
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if (likely(update_fast_ctr(brw, +1)))
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return;
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/* Avoid rwsem_acquire_read() and rwsem_release() */
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__down_read(&brw->rw_sem);
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atomic_inc(&brw->slow_read_ctr);
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__up_read(&brw->rw_sem);
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}
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EXPORT_SYMBOL_GPL(percpu_down_read);
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int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
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{
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if (unlikely(!update_fast_ctr(brw, +1))) {
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if (!__down_read_trylock(&brw->rw_sem))
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return 0;
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atomic_inc(&brw->slow_read_ctr);
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__up_read(&brw->rw_sem);
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}
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rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
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return 1;
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}
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void percpu_up_read(struct percpu_rw_semaphore *brw)
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{
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rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
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if (likely(update_fast_ctr(brw, -1)))
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return;
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/* false-positive is possible but harmless */
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if (atomic_dec_and_test(&brw->slow_read_ctr))
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wake_up_all(&brw->write_waitq);
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}
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EXPORT_SYMBOL_GPL(percpu_up_read);
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static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
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{
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unsigned int sum = 0;
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int cpu;
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for_each_possible_cpu(cpu) {
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sum += per_cpu(*brw->fast_read_ctr, cpu);
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per_cpu(*brw->fast_read_ctr, cpu) = 0;
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}
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return sum;
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}
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void percpu_down_write(struct percpu_rw_semaphore *brw)
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{
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/*
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* Make rcu_sync_is_idle() == F and thus disable the fast-path in
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* percpu_down_read() and percpu_up_read(), and wait for gp pass.
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*
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* The latter synchronises us with the preceding readers which used
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* the fast-past, so we can not miss the result of __this_cpu_add()
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* or anything else inside their criticial sections.
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*/
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rcu_sync_enter(&brw->rss);
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/* exclude other writers, and block the new readers completely */
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down_write(&brw->rw_sem);
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/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
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atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
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/* wait for all readers to complete their percpu_up_read() */
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wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
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}
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EXPORT_SYMBOL_GPL(percpu_down_write);
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void percpu_up_write(struct percpu_rw_semaphore *brw)
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{
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/* release the lock, but the readers can't use the fast-path */
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up_write(&brw->rw_sem);
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/*
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* Enable the fast-path in percpu_down_read() and percpu_up_read()
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* but only after another gp pass; this adds the necessary barrier
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* to ensure the reader can't miss the changes done by us.
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*/
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rcu_sync_exit(&brw->rss);
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}
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EXPORT_SYMBOL_GPL(percpu_up_write);
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