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Date:	Mon, 1 Sep 2014 10:38:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [Patch] perf_event: fix a race condition in
 perf_remove_from_context()

On Thu, Aug 28, 2014 at 04:27:35PM -0700, Cong Wang wrote:
> From: Cong Wang <cwang@...pensource.com>
> 
> We saw a kernel soft lockup in perf_remove_from_context(),
> it looks like the `perf` process, when exiting, could not go
> out of the retry loop. Meanwhile, the target process was forking
> a child. So either the target process should execute the smp
> function call to deactive the event (if it was running) or it should
> do a context switch which deactives the event.
> 
> It seems we optimize out a context switch in perf_event_context_sched_out(),
> and what's more important, we still test an obsolete task pointer when
> retrying, so no one actually would deactive that event in this situation.
> Fix it directly by reloading the task pointer in perf_remove_from_context().
> This should fix the above soft lockup.



> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f9c1ed0..c4141a0 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1524,6 +1524,11 @@ retry:

Please use either:

.gitconfig:

[diff "default"]
        xfuncname = "^[[:alpha:]$_].*[^:]$"

.quiltrc:

QUILT_DIFF_OPTS="-F ^[[:alpha:]\$_].*[^:]\$"

>  	 */
>  	if (ctx->is_active) {
>  		raw_spin_unlock_irq(&ctx->lock);
> +		/*
> +		 * Reload the task pointer, it might have been changed by
> +		 * a concurrent perf_event_context_sched_out() without switching
> +		 */
> +		task = ctx->task;
>  		goto retry;
>  	}

You forgot to check if that same error happened in other places (it
does), please fix all of them.
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