linux/kernel/locking/qrwlock.c
Waiman Long 28ce0e70ec locking/qrwlock: Cleanup queued_write_lock_slowpath()
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
2021-05-06 15:33:49 +02:00

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);