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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
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/*
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* Queued read/write locks
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*
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* (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
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*
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* Authors: Waiman Long <waiman.long@hp.com>
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*/
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#include <linux/smp.h>
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#include <linux/bug.h>
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#include <linux/cpumask.h>
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#include <linux/percpu.h>
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#include <linux/hardirq.h>
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#include <linux/spinlock.h>
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/**
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* queued_read_lock_slowpath - acquire read lock of a queue rwlock
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* @lock: Pointer to queue rwlock structure
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*/
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void queued_read_lock_slowpath(struct qrwlock *lock)
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{
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/*
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* Readers come here when they cannot get the lock without waiting
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*/
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if (unlikely(in_interrupt())) {
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/*
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* Readers in interrupt context will get the lock immediately
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* if the writer is just waiting (not holding the lock yet),
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* so spin with ACQUIRE semantics until the lock is available
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* without waiting in the queue.
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*/
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atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
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return;
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}
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atomic_sub(_QR_BIAS, &lock->cnts);
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/*
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* Put the reader into the wait queue
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*/
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arch_spin_lock(&lock->wait_lock);
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atomic_add(_QR_BIAS, &lock->cnts);
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/*
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* The ACQUIRE semantics of the following spinning code ensure
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* that accesses can't leak upwards out of our subsequent critical
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* section in the case that the lock is currently held for write.
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*/
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atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
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/*
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* Signal the next one in queue to become queue head
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*/
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arch_spin_unlock(&lock->wait_lock);
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}
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EXPORT_SYMBOL(queued_read_lock_slowpath);
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/**
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* queued_write_lock_slowpath - acquire write lock of a queue rwlock
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* @lock : Pointer to queue rwlock structure
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*/
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void queued_write_lock_slowpath(struct qrwlock *lock)
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{
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int cnts;
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/* Put the writer into the wait queue */
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arch_spin_lock(&lock->wait_lock);
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/* Try to acquire the lock directly if no reader is present */
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if (!(cnts = atomic_read(&lock->cnts)) &&
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atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
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goto unlock;
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/* Set the waiting flag to notify readers that a writer is pending */
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atomic_or(_QW_WAITING, &lock->cnts);
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/* When no more readers or writers, set the locked flag */
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do {
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cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
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} while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
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unlock:
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arch_spin_unlock(&lock->wait_lock);
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}
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EXPORT_SYMBOL(queued_write_lock_slowpath);
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