linux-kernel - Re: [PATCH v3 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW

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Message-ID: <20150731083920.GD19282@twins.programming.kicks-ass.net>
Date:	Fri, 31 Jul 2015 10:39:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v3 1/7] locking/pvqspinlock: Unconditional PV kick with
 _Q_SLOW_VAL

On Wed, Jul 22, 2015 at 04:12:36PM -0400, Waiman Long wrote:
> The smp_store_release() is not a full barrier. In order to avoid missed
> wakeup, we may need to add memory barrier around locked and cpu state
> variables adding to complexity. As the chance of spurious wakeup is very
> low, it is easier and safer to just do an unconditional kick at unlock
> time.

This needs to better spell out the race; my still sleeping brain doesn't
want to co-operate and its generally better to spell out these things
anyway.
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