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Date: Fri, 30 Aug 2002 09:06:38 +0200 (CEST)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: Christian Ehrhardt <ehrhardt@mathematik.uni-ulm.de>
Cc: Burton Windle <bwindle@fint.org>,
       Kernel Mailing List <linux-kernel@vger.kernel.org>,
       <kpreempt-tech@lists.sourceforge.net>, <rml@tech9.net>
Subject: Re: 2.5.32: DEBUG_SLAB and PREEMPT = constant oops in schedule()
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On Thu, 29 Aug 2002, Christian Ehrhardt wrote:

> sys_sched_setaffinity, lines:
>    1571         get_task_struct(p);
>    1572         read_unlock(&tasklist_lock);

> Line 1571 calls get_task_struct because the task might exit during the
> syscall. Suppose that this is exactly what happens. This means that line
> 1583 will effectivly free the task. But set_cpus_allowed stuffed a
> pointer to the task into a request struct without incrementing the usage
> count of the task struct.

note that the scenario you describe is not possible, because
set_cpus_allowed() will wait for the migration thread to do its work - so
the put_task_struct can never come before the last use of the task
structure. See the 'down(&req.sem)' in set_cpus_allowed(), and the
up(&req->sem) in migration_thread().

	Ingo

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