linux-kernel - Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock

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Message-ID: <1265042795.29013.58.camel@gandalf.stny.rr.com>
Date:	Mon, 01 Feb 2010 11:46:35 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [patch 2/3] scheduler: add full memory barriers upon task
 switch at runqueue lock/unlock

[ Added linux-arch ]

On Mon, 2010-02-01 at 17:29 +0100, Peter Zijlstra wrote:
> On Mon, 2010-02-01 at 11:24 -0500, Steven Rostedt wrote:
> > 
> > Doesn't set_bit imply a wmb()? If so couldn't we do:
> 
> Nope, that's what we have smp_mb__{before,after}_set_bit() for.
> 
> On x86 its a LOCK'ed op, so sure it implies a full membarrier, but not
> in generic.
> 
> on x86 we have plenty serializing instructions before and after rq->curr
> is set so none of the crazyness is needed at all. The only thing is !
> x86.
> 

So if we go with Linus's approach and make all archs guarantee that
switch_mm() implies a full mb(), then we can simplify the
sys_membarrier() code?

That looks like the best approach.

-- Steve



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