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locking/qspinlock: Use smp_cond_load_relaxed() to wait for next node 

When a locker reaches the head of the queue and takes the lock, a
concurrent locker may enqueue and force the lock holder to spin
whilst its node->next field is initialised. Rather than open-code
a READ_ONCE/cpu_relax() loop, this can be implemented using
smp_cond_load_relaxed() instead.

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/1524738868-31318-10-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 11:34:23 +01:00 committed by Ingo Molnar
parent 7f56b58a92
commit c131a198c4
1 changed files with 2 additions and 4 deletions
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6
kernel/locking/qspinlock.c
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@ -483,10 +483,8 @@ locked:
/*
* contended path; wait for next if not observed yet, release.
*/
if (!next) {
while (!(next = READ_ONCE(node->next)))
cpu_relax();
}
if (!next)
next = smp_cond_load_relaxed(&node->next, (VAL));
arch_mcs_spin_unlock_contended(&next->locked);
pv_kick_node(lock, next);