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locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() when locking

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When reaching the head of an uncontended queue on the qspinlock slow-path,
using a try_cmpxchg() instead of a cmpxchg() operation to transition the
lock work to _Q_LOCKED_VAL generates slightly better code for x86 and
pretty much identical code for arm64.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1524738868-31318-13-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon 2018-04-26 11:34:26 +01:00 committed by Ingo Molnar
parent 9d4646d14d
commit ae75d9089f
1 changed files with 9 additions and 10 deletions
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9
kernel/locking/qspinlock.c
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@ -467,16 +467,15 @@ locked:
* Otherwise, we only need to grab the lock.
*/
/* In the PV case we might already have _Q_LOCKED_VAL set */
if ((val & _Q_TAIL_MASK) == tail) {
/*
* In the PV case we might already have _Q_LOCKED_VAL set.
*
* The atomic_cond_read_acquire() call above has provided the
* necessary acquire semantics required for locking.
*/
old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
if (((val & _Q_TAIL_MASK) == tail) &&
atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
goto release; /* No contention */
}
/* Either somebody is queued behind us or _Q_PENDING_VAL is set */
set_locked(lock);