linux-kernel - Re: [RFC][PATCH] sched: Fix race in task

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Message-ID: <20120626201339.GU3869@google.com>
Date:	Tue, 26 Jun 2012 13:13:39 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stefan Bader <stefan.bader@...onical.com>, mingo@...nel.org,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Mike Galbraith <efault@....de>,
	Andrew Vagin <avagin@...nvz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] sched: Fix race in task_group()

On Tue, Jun 26, 2012 at 03:48:35PM +0200, Peter Zijlstra wrote:
> Here's one that's actually compile tested (with the right CONFIG_foo
> enabled) and I fixed the autogroup lockdep splat.
> 
> ---
> Subject: sched: Fix race in task_group()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Fri, 22 Jun 2012 13:36:05 +0200
> 
> Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
> call task_group() too many times in set_task_rq()"), he found the reason
> to be that the multiple task_group() invocations in set_task_rq()
> returned different values.

Hmm... short of intertwining locking further I don't think we can
solve this in prettier way.  So, yeah, looks good to me from cgroup
POV.

> Looking at all that I found a lack of serialization and plain wrong
> comments.
> 
> The below tries to fix it using an extra pointer which is updated under
> the appropriate scheduler locks. Its not pretty, but I can't really see
> another way given how all the cgroup stuff works.

BTW your patch is whitespace broken.  Seems like QP encoded.

Thanks.

-- 
tejun
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