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locking/qspinlock: Avoid redundant read of next pointer

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With optimistic prefetch of the next node cacheline, the next pointer
may have been properly inititalized. As a result, the reading
of node->next in the contended path may be redundant. This patch
eliminates the redundant read if the next pointer value is not NULL.

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/1447114167-47185-4-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 19:09:23 -05:00 • committed by Ingo Molnar
parent 81b5598665
commit aa68744f80
1 changed files with 6 additions and 3 deletions
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9
kernel/locking/qspinlock.c
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@ -396,6 +396,7 @@ queue:
* p,*,* -> n,*,* * p,*,* -> n,*,*
*/ */
old = xchg_tail(lock, tail); old = xchg_tail(lock, tail);
next = NULL;
/* /*
* if there was a previous node; link it and wait until reaching the * if there was a previous node; link it and wait until reaching the
@ -463,10 +464,12 @@ queue:
} }
/* /*
* contended path; wait for next, release. * contended path; wait for next if not observed yet, release.
*/ */
while (!(next = READ_ONCE(node->next))) if (!next) {
cpu_relax(); while (!(next = READ_ONCE(node->next)))
cpu_relax();
}
arch_mcs_spin_unlock_contended(&next->locked); arch_mcs_spin_unlock_contended(&next->locked);
pv_kick_node(lock, next); pv_kick_node(lock, next);