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Date: Mon, 9 Nov 2015 19:09:23 -0500 From: Waiman Long <Waiman.Long@....com> To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, "H. Peter Anvin" <hpa@...or.com> Cc: x86@...nel.org, linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@....com>, Douglas Hatch <doug.hatch@....com>, Davidlohr Bueso <dave@...olabs.net>, Waiman Long <Waiman.Long@....com> Subject: [PATCH tip/locking/core v10 3/7] locking/qspinlock: Avoid redundant read of next pointer 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@....com> --- kernel/locking/qspinlock.c | 9 ++++++--- 1 files changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 365b203..9862078 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -396,6 +396,7 @@ queue: * p,*,* -> n,*,* */ old = xchg_tail(lock, tail); + next = NULL; /* * 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))) - cpu_relax(); + if (!next) { + while (!(next = READ_ONCE(node->next))) + cpu_relax(); + } arch_mcs_spin_unlock_contended(&next->locked); pv_kick_node(lock, next); -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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