linux-kernel - Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts

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Message-Id: <1243000016.6582.574.camel@laptop>
Date:	Fri, 22 May 2009 15:46:56 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Mackerras <paulus@...ba.org>
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 2/2] perf_counter: optimize context switch between
 identical inherited contexts

On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:
> +       next_ctx = next->perf_counter_ctxp;
> +       if (next_ctx && context_equiv(ctx, next_ctx)) {
> +               task->perf_counter_ctxp = next_ctx;
> +               next->perf_counter_ctxp = ctx;
> +               ctx->task = next;
> +               next_ctx->task = task;
> +               return;
> +       }

Ingo just pointed out that there is nothing there to close the race with
attaching a counter.

That is, you could end up attaching your counter to the wrong task.

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