linux/kernel/locking/qrwlock.c
Will Deacon d133166146 locking/qrwlock: Prevent slowpath writers getting held up by fastpath
When a prospective writer takes the qrwlock locking slowpath due to the
lock being held, it attempts to cmpxchg the wmode field from 0 to
_QW_WAITING so that concurrent lockers also take the slowpath and queue
on the spinlock accordingly, allowing the lockers to drain.

Unfortunately, this isn't fair, because a fastpath writer that comes in
after the lock is made available but before the _QW_WAITING flag is set
can effectively jump the queue. If there is a steady stream of prospective
writers, then the waiter will be held off indefinitely.

This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
into two distinct fields: _QW_LOCKED continues to occupy the bottom byte
of the lockword so that it can be cleared unconditionally when unlocking,
but _QW_WAITING now occupies what used to be the bottom bit of the reader
count. This then forces the slow-path for concurrent lockers.

Tested-by: Waiman Long <longman@redhat.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Adam Wallis <awallis@codeaurora.org>
Tested-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jeremy.Linton@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1507810851-306-6-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-10-25 10:57:25 +02:00

93 lines
2.8 KiB
C

/*
* Queued read/write locks
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* (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>
#include <asm/qrwlock.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)
{
/* 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 (!atomic_read(&lock->cnts) &&
(atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
goto unlock;
/* Set the waiting flag to notify readers that a writer is pending */
atomic_add(_QW_WAITING, &lock->cnts);
/* When no more readers or writers, set the locked flag */
do {
atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
_QW_LOCKED) != _QW_WAITING);
unlock:
arch_spin_unlock(&lock->wait_lock);
}
EXPORT_SYMBOL(queued_write_lock_slowpath);