linux-kernel - Re: regression introduced by - timers: fix itimer/many thread hang

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Date:	Mon, 17 Nov 2008 15:36:23 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Frank Mayhar <fmayhar@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Doug Chapman <doug.chapman@...com>, mingo@...e.hu,
	adobriyan@...il.com, akpm@...ux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression introduced by - timers: fix itimer/many thread hang

On 11/14, Oleg Nesterov wrote:
>
> Currently I am trying to find the ugly, but simple fixes for 2.6.28.
>
> account_group_user_time(), run_posix_cpu_timers() are simpler to
> fix. Again, I need to actually read the code, but afaics we can
> rely on the fact that the task is current, so we can change the
> code
>
> 	-	if (!->signal)
> 	+	if (->exit_state)
> 			return;

Yes, unless I missed something again, this should work. I'll send
the (simple) patches soon, but I have no idea how to test them.

However, I'm afraid there is another problem. On 32 bit cpus we can't
read "u64 sum_exec_runtime" atomically, and so thread_group_cputime()
can "overestimate" ->sum_exec_runtime by (up to) UINT_MAX if it races
with the thread which updates its per_cpu_ptr(.totals). This for example
means that check_process_timers() can fire the CPUCLOCK_SCHED timers
before time.

No?

Oleg.

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