linux-kernel - Re: [PATCH 1/2] perf_counter: dynamically allocate tasks' perf_counter

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Message-ID: <18966.28834.891679.726583@cargo.ozlabs.ibm.com>
Date:	Fri, 22 May 2009 19:30:10 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf_counter: dynamically allocate tasks'
 perf_counter_context struct [v2]

Peter Zijlstra writes:

> On Fri, 2009-05-22 at 14:17 +1000, Paul Mackerras wrote:
> > +               /*
> > +                * Make sure other cpus see correct values for *ctx
> > +                * once task->perf_counter_ctxp is visible to them.
> > +                */
> > +               smp_wmb();
> > +               tctx = cmpxchg(&task->perf_counter_ctxp, NULL, ctx);
> 
> 
> Documentation/memory-barriers.txt:
> 
> > Any atomic operation that modifies some state in memory and returns information
> > about the state (old or new) implies an SMP-conditional general memory barrier
> > (smp_mb()) on each side of the actual operation (with the exception of
> > explicit lock operations, described later).  These include:
> 
> >         cmpxchg();

OK, fair enough, I'll take it out.

Paul.
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