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Date: Thu, 20 Feb 2003 20:00:26 +0100 (CET)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: Zwane Mwaikambo <zwane@holomorphy.com>, Chris Wedgwood <cw@f00f.org>,
       Kernel Mailing List <linux-kernel@vger.kernel.org>,
       "Martin J. Bligh" <mbligh@aracnet.com>,
       William Lee Irwin III <wli@holomorphy.com>
Subject: Re: doublefault debugging (was Re: Linux v2.5.62 --- spontaneous
 reboots)
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On Thu, 20 Feb 2003, Linus Torvalds wrote:

> > a true heisenbug. I cannot reproduce it anymore. Anyway, from the serial
> > console i collected 3 instances of crashes - whatever it's worth.
> 
> Pretty much every single time, release_task() has been there on the
> backtrace.
> 
> In fact, I bet you this code in do_exit() is the cause:
> 
>         preempt_disable();
> 
>         if (tsk->exit_signal == -1)
> ***             release_task(tsk);	***
> 
>         schedule();
> 
> Note how "release_task()" will be releasing the stack that the process
> is running on right now. [...]

but, release_task() is a delayed thing for exactly this reason. It fills
out the per-CPU task_cache but does not free the task.

the release_task() + schedule() must be atomic though - ie. we must not be
preempted anytime inbetween [because that other task could free the
task_cache] - but i wasnt running with CONFIG_PREEMPT, so i cannot see how
it could happen.

	Ingo

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