linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

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Message-ID: <1262883604.28171.3733.camel@gandalf.stny.rr.com>
Date:	Thu, 07 Jan 2010 12:00:04 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	tglx@...utronix.de, peterz@...radead.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier

On Thu, 2010-01-07 at 08:49 -0800, Paul E. McKenney wrote:

> > If the task->rq changes, or the task->rq->curr changes, and makes the
> > condition fail (or even pass), the events that cause those changes are
> > probably good enough than needing to call smp_mb();
> 
> This narrows the fatal window, but does not eliminate it.  :-(
> 
> The CPU doing the sys_membarrier() might see an old value of ->curr,
> and the other CPU might see an old value of whatever pointer we are
> trying to recycle.  This combination is fatal.

But for curr to change, the rq spin lock must have been held. Which
implies smp_wb(). I would think that if you do a smp_rb() wouldn't that
guarantee that you see the new value of curr?

-- Steve


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