linux-kernel - Re: [PATCH] posix-cpu-timers: workaround to suppress the problems with mt exec

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Date:	Tue, 9 Nov 2010 15:54:20 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Roland McGrath <roland@...hat.com>, stable@...nel.org
Subject: Re: [PATCH] posix-cpu-timers: workaround to suppress the problems
 with mt exec

On Fri, Nov 05, 2010 at 04:53:42PM +0100, Oleg Nesterov wrote:
> posix-cpu-timers.c correctly assumes that the dying process does
> posix_cpu_timers_exit_group() and removes all !CPUCLOCK_PERTHREAD
> timers from signal->cpu_timers list.
> 
> But, it also assumes that timer->it.cpu.task is always the group
> leader, and thus the dead ->task means the dead thread group.
> 
> This is obviously not true after de_thread() changes the leader.
> After that almost every posix_cpu_timer_ method has problems.
> 
> It is not simple to fix this bug correctly. First of all, I think
> that timer->it.cpu should use struct pid instead of task_struct.
> Also, the locking should be reworked completely. In particular,
> tasklist_lock should not be used at all. This all needs a lot of
> nontrivial and hard-to-test changes.
> 
> Change __exit_signal() to do posix_cpu_timers_exit_group() when
> the old leader dies during exec. This is not the fix, just the
> temporary hack to hide the problem for 2.6.37 and stable. IOW,
> this is obviously wrong but this is what we currently have anyway:
> cpu timers do not work after mt exec.
> 
> In theory this change adds another race. The exiting leader can
> detach the timers which were attached to the new leader. However,
> the window between de_thread() and release_task() is small, we
> can pretend that sys_timer_create() was called before de_thread().
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Reviewed-by: Stanislaw Gruszka <sgruszka@...hat.com>

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