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>

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 56af1fa..d6c3b02 100644 (file)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -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);