linux-kernel - Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access

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Message-ID: <20100329215138.GP2569@linux.vnet.ibm.com>
Date:	Mon, 29 Mar 2010 14:51:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, abogani@...ware.it, menage@...gle.com,
	lizf@...fujitsu.com
Subject: Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access

On Mon, Mar 29, 2010 at 11:42:55PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-03-29 at 14:34 -0700, Paul E. McKenney wrote:
> 
> > And it appears that my patch is at best insufficient:
> > http://paste.ubuntu.com/406189/
> > 
> > Left to myself, I would wrap copy_process() with rcu_read_lock(),
> > but I would rather hear your thoughts before doing too much more
> > semi-random hacking.  ;-)
> 
> Well, I don't think you can get away with that, copy_process() wants to
> sleep on quite a few places ;-) Also, locks should be taken at the
> smallest possible scope, unless we want to go back to BKL style
> locking :-)

No argument here!  ;-)

> As to that freezer splat, you'd have to chase down the cgroup folks, I'm
> fully ignorant on that.

K, adding them to CC.  The two splats are:

	http://pastebin.ubuntu.com/406131/
	http://paste.ubuntu.com/406189/

Some additional RCU protection is required, or perhaps some suppression
of false positives.  Thoughts?

> For the set_task_rq() one, I'm afraid someone (which again I'm afraid
> will be me) will have to look into how the task_group muck ties into the
> cgroup muck as I think the original authors of that ran off :/

Sigh!

							Thanx, Paul
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