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Subject: Re: [PATCH 2/2] perf_counter: optimize context switch between
 identical inherited contexts
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Corey Ashford <cjashfor@linux.vnet.ibm.com>,
       Thomas Gleixner <tglx@linutronix.de>
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Date: Fri, 22 May 2009 15:46:56 +0200
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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.