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Date: Wed, 23 Mar 2011 13:47:19 +0100
Message-ID: <AANLkTimC6MqioV=6Ry=xvL9m4NeAEmZbkoOGQZ+66Duc@mail.gmail.com>
Subject: Re: [PATCH] perf_events: fix cgrp stale pointer in update_cgrp_time_from_cpuctx()
From: Stephane Eranian <eranian@google.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
        perfmon2-devel@lists.sf.net, paulus@samba.org, davem@davemloft.net
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On Wed, Mar 23, 2011 at 12:36 PM, Ingo Molnar <mingo@elte.hu> wrote:
>
> * Stephane Eranian <eranian@google.com> wrote:
>
>> -     if (is_cgroup_event(event))
>> +     if (is_cgroup_event(event)) {
>>               ctx->nr_cgroups--;
>> +             cpuctx = __get_cpu_context(ctx);
>> +             /*
>> +              * if there are no more cgroup events
>> +              * then clear cgrp to avoid stale pointer
>> +              * in update_cgrp_time_from_cpuctx()
>> +              */
>> +             if (!ctx->nr_cgroups)
>> +                     cpuctx->cgrp = NULL;
>> +     }
>
> The ->cgrp pointer does not exist on !CGROUPS kernels. I suspect the cleanest
> approach would be to make those two cgrp fields available unconditionally in
> struct perf_event.
>
Argh, I forgot to test !CGROUP.
I tend to agree with you on making those fields unconditional. We
don't save that
much by not doing it.

> Thanks,
>
>        Ingo
>