linux-kernel - Re: [regression] Crash in wb_clear

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Date:	Tue, 6 Jul 2010 21:47:34 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bill Davidsen <davidsen@....com>, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [regression] Crash in wb_clear_pending()

On Tue, Jul 06, 2010 at 08:47:33AM +0200, Ingo Molnar wrote:
> Yeah, i agree - especially since the smaller patch is still pretty large (not 
> a oneliner), plus it does not appear that the precise failure mode is fully 
> understood either.

The two failure modes we saw are very precicely understood:

 - access to ->state after it's been freed
 - access to ->state after the caller which has the bdi_work struct on
   stack has been woken up.

But I seriously fear that there are even more dragons hidden in this
maze of bit wakeups and rcu, so replacing it with a proper completion
seems by far safer to me.

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