linux-kernel - Re: [PATCH 2.6.27-rc5] Fix itimer/many thread hang.

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Date:	Wed, 10 Sep 2008 15:44:01 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Frank Mayhar <fmayhar@...gle.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2.6.27-rc5] Fix itimer/many thread hang.

On 09/09, Roland McGrath wrote:
>
> > As for this particular function, it seems to me that ->signal == NULL
> > is not possible, no?
> 
> The concern is to make sure that the timer tick path is always safe for a
> tick that hits during release_task(current) in exit_notify().  A timer tick
> there can come with current->signal == NULL.  We just need to make sure one
> way or another that this is safe.

run_posix_cpu_timers() must check ->signal != NULL anyway. (The same for
other functions like account_group_user_time() which are called by the
timer tick).

Apart from the timer tick, "current" should not use this function after
exit_notify().

And, if tsk != current, ->signal must be pinned, this means it can't be NULL.

Perhaps I missed something, but imho this check is confusing and misleading.
However, this is just a minor detail even if I am right.

Oleg.

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