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locking/qspinlock: Use _acquire/_release() versions of cmpxchg() & xchg() 

This patch replaces the cmpxchg() and xchg() calls in the native
qspinlock code with the more relaxed _acquire or _release versions of
those calls to enable other architectures to adopt queued spinlocks
with less memory barrier performance overhead.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447114167-47185-2-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Waiman Long 2015-11-09 19:09:21 -05:00 committed by Ingo Molnar
parent 978e5a3692
commit 64d816cba0
2 changed files with 29 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
include/asm-generic/qspinlock.h
View File

@ -12,8 +12,9 @@
* GNU General Public License for more details.
*
* (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
* (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
*
* Authors: Waiman Long <waiman.long@hp.com>
* Authors: Waiman Long <waiman.long@hpe.com>
*/
#ifndef __ASM_GENERIC_QSPINLOCK_H
#define __ASM_GENERIC_QSPINLOCK_H
@ -62,7 +63,7 @@ static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
static __always_inline int queued_spin_trylock(struct qspinlock *lock)
{
if (!atomic_read(&lock->val) &&
(atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0))
(atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
return 1;
return 0;
}
@ -77,7 +78,7 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock)
{
u32 val;
val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
if (likely(val == 0))
return;
queued_spin_lock_slowpath(lock, val);
@ -93,7 +94,7 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
/*
* smp_mb__before_atomic() in order to guarantee release semantics
*/
smp_mb__before_atomic_dec();
smp_mb__before_atomic();
atomic_sub(_Q_LOCKED_VAL, &lock->val);
}
#endif

29
kernel/locking/qspinlock.c
View File

@ -14,8 +14,9 @@
* (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
* (C) Copyright 2013-2014 Red Hat, Inc.
* (C) Copyright 2015 Intel Corp.
* (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
*
* Authors: Waiman Long <waiman.long@hp.com>
* Authors: Waiman Long <waiman.long@hpe.com>
* Peter Zijlstra <peterz@infradead.org>
*/
@ -176,7 +177,12 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
{
struct __qspinlock *l = (void *)lock;
return (u32)xchg(&l->tail, tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
/*
* Use release semantics to make sure that the MCS node is properly
* initialized before changing the tail code.
*/
return (u32)xchg_release(&l->tail,
tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
}
#else /* _Q_PENDING_BITS == 8 */
@ -208,7 +214,11 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
for (;;) {
new = (val & _Q_LOCKED_PENDING_MASK) | tail;
old = atomic_cmpxchg(&lock->val, val, new);
/*
* Use release semantics to make sure that the MCS node is
* properly initialized before changing the tail code.
*/
old = atomic_cmpxchg_release(&lock->val, val, new);
if (old == val)
break;
@ -319,7 +329,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
if (val == new)
new |= _Q_PENDING_VAL;
old = atomic_cmpxchg(&lock->val, val, new);
/*
* Acquire semantic is required here as the function may
* return immediately if the lock was free.
*/
old = atomic_cmpxchg_acquire(&lock->val, val, new);
if (old == val)
break;
@ -426,7 +440,12 @@ queue:
set_locked(lock);
break;
}
old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
/*
* The smp_load_acquire() call above has provided the necessary
* acquire semantics required for locking. At most two
* iterations of this loop may be ran.
*/
old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
goto release; /* No contention */