mirror of
https://github.com/torvalds/linux.git
synced 2024-12-06 11:01:43 +00:00
28ce0e70ec
Make the code more readable by replacing the atomic_cmpxchg_acquire() by an equivalent atomic_try_cmpxchg_acquire() and change atomic_add() to atomic_or(). For architectures that use qrwlock, I do not find one that has an atomic_add() defined but not an atomic_or(). I guess it should be fine by changing atomic_add() to atomic_or(). Note that the previous use of atomic_add() isn't wrong as only one writer that is the wait_lock owner can set the waiting flag and the flag will be cleared later on when acquiring the write lock. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Will Deacon <will@kernel.org> Link: https://lkml.kernel.org/r/20210426185017.19815-1-longman@redhat.com
84 lines
2.3 KiB
C
84 lines
2.3 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
|
|
/*
|
|
* Queued read/write locks
|
|
*
|
|
* (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
|
|
*
|
|
* Authors: Waiman Long <waiman.long@hp.com>
|
|
*/
|
|
#include <linux/smp.h>
|
|
#include <linux/bug.h>
|
|
#include <linux/cpumask.h>
|
|
#include <linux/percpu.h>
|
|
#include <linux/hardirq.h>
|
|
#include <linux/spinlock.h>
|
|
|
|
/**
|
|
* queued_read_lock_slowpath - acquire read lock of a queue rwlock
|
|
* @lock: Pointer to queue rwlock structure
|
|
*/
|
|
void queued_read_lock_slowpath(struct qrwlock *lock)
|
|
{
|
|
/*
|
|
* Readers come here when they cannot get the lock without waiting
|
|
*/
|
|
if (unlikely(in_interrupt())) {
|
|
/*
|
|
* Readers in interrupt context will get the lock immediately
|
|
* if the writer is just waiting (not holding the lock yet),
|
|
* so spin with ACQUIRE semantics until the lock is available
|
|
* without waiting in the queue.
|
|
*/
|
|
atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
|
|
return;
|
|
}
|
|
atomic_sub(_QR_BIAS, &lock->cnts);
|
|
|
|
/*
|
|
* Put the reader into the wait queue
|
|
*/
|
|
arch_spin_lock(&lock->wait_lock);
|
|
atomic_add(_QR_BIAS, &lock->cnts);
|
|
|
|
/*
|
|
* The ACQUIRE semantics of the following spinning code ensure
|
|
* that accesses can't leak upwards out of our subsequent critical
|
|
* section in the case that the lock is currently held for write.
|
|
*/
|
|
atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
|
|
|
|
/*
|
|
* Signal the next one in queue to become queue head
|
|
*/
|
|
arch_spin_unlock(&lock->wait_lock);
|
|
}
|
|
EXPORT_SYMBOL(queued_read_lock_slowpath);
|
|
|
|
/**
|
|
* queued_write_lock_slowpath - acquire write lock of a queue rwlock
|
|
* @lock : Pointer to queue rwlock structure
|
|
*/
|
|
void queued_write_lock_slowpath(struct qrwlock *lock)
|
|
{
|
|
int cnts;
|
|
|
|
/* Put the writer into the wait queue */
|
|
arch_spin_lock(&lock->wait_lock);
|
|
|
|
/* Try to acquire the lock directly if no reader is present */
|
|
if (!(cnts = atomic_read(&lock->cnts)) &&
|
|
atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
|
|
goto unlock;
|
|
|
|
/* Set the waiting flag to notify readers that a writer is pending */
|
|
atomic_or(_QW_WAITING, &lock->cnts);
|
|
|
|
/* When no more readers or writers, set the locked flag */
|
|
do {
|
|
cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
|
|
} while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
|
|
unlock:
|
|
arch_spin_unlock(&lock->wait_lock);
|
|
}
|
|
EXPORT_SYMBOL(queued_write_lock_slowpath);
|