linux-kernel - Re: [PATCH] balance RT task resched only on runqueue

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Date:	Wed, 5 Mar 2008 16:24:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sripathi Kodi <sripathik@...ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] balance RT task resched only on runqueue


* Steven Rostedt <rostedt@...dmis.org> wrote:

> In the RT kernel we discovered a place that can reschedule a task 
> without holding the tasks runqueue lock.  This was caused by the RT 
> balancing code that pulls RT tasks to the current run queue and will 
> reschedule the current task.
> 
> There's a slight chance that the pulling of the RT tasks will release 
> the current runqueue's lock and retake it (in the 
> double_lock_balance). During this time that the runqueue is released, 
> the current task can migrate to another runqueue.
> 
> In the prio_changed_rt code, after the pull, if the current task is of 
> lesser priority than one of the RT tasks pulled, resched_task is 
> called on the current task. If the current task had migrated in that 
> small window, resched_task will be called without holding the runqueue 
> lock for the runqueue that the task is on.
> 
> This race condition also exists in the mainline kernel and this patch 
> adds a check to make sure the task hasn't migrated before calling 
> resched_task.

thanks Steve, applied.

	Ingo
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