mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-28 03:15:23 +07:00
d133166146
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>
93 lines
2.8 KiB
C
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);
|