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Date: Mon, 23 Nov 2015 08:26:52 -0800
From: tip-bot for Waiman Long <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: scott.norton@....com, Waiman.Long@....com, hpa@...or.com,
dave@...olabs.net, mingo@...nel.org, peterz@...radead.org,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
tglx@...utronix.de, paulmck@...ux.vnet.ibm.com,
torvalds@...ux-foundation.org, doug.hatch@....com
Subject: [tip:locking/core] locking/qspinlock: Use _acquire/_release()
versions of cmpxchg() & xchg()
Commit-ID: 64d816cba06c67eeee455b8c78ebcda349d49c24
Gitweb: http://git.kernel.org/tip/64d816cba06c67eeee455b8c78ebcda349d49c24
Author: Waiman Long <Waiman.Long@....com>
AuthorDate: Mon, 9 Nov 2015 19:09:21 -0500
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 23 Nov 2015 10:01:58 +0100
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@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: Douglas Hatch <doug.hatch@....com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Scott J Norton <scott.norton@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1447114167-47185-2-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
include/asm-generic/qspinlock.h | 9 +++++----
kernel/locking/qspinlock.c | 29 ++++++++++++++++++++++++-----
2 files changed, 29 insertions(+), 9 deletions(-)
diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index e2aadbc..39e1cb2 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -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@...com>
+ * Authors: Waiman Long <waiman.long@....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
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 87e9ce6a..7868418 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -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@...com>
+ * Authors: Waiman Long <waiman.long@....com>
* Peter Zijlstra <peterz@...radead.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 */
--
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