linux-kernel - Re: [PATCH 1/1] perf_event: add check for group

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Date:	Wed, 30 Dec 2009 13:28:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Liming Wang <liming.wang@...driver.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] perf_event: add check for group_list if the parent
 isn't counted

On Wed, 2009-12-30 at 19:28 +0800, Liming Wang wrote:
> If the parent has no entry in group_list, child_ctx will not be
> allocated, which will lead dereference of a NULL child_ctx.

That changelog sucks.

Best I can make of it is that there is a race where the parent gets his
context instantiated and we manage to get the mutex before the other
thread manages to add the first event.

Then we observe parent_event_ctx but have an empty list.

Is that it?

In that case, would it not be better to change the if (inherited_all)
condition to if (inherited_all && child_ctx) ?

> Signed-off-by: Liming Wang <liming.wang@...driver.com>
> ---
>  kernel/perf_event.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 5b987b4..3664c4b 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -5126,6 +5126,8 @@ int perf_event_init_task(struct task_struct *child)
>  	 */
>  	mutex_lock(&parent_ctx->mutex);
>  
> +	if (list_empty(&parent_ctx->group_list))
> +		goto exit;
>  	/*
>  	 * We dont have to disable NMIs - we are only looking at
>  	 * the list, not manipulating it:


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