linux-kernel - Re: [PATCH RESEND 2/3 v2.6.39-rc7] block: make disk_block_events() properly wait for work cancellation

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Message-ID: <20110518110732.GW20624@htj.dyndns.org>
Date:	Wed, 18 May 2011 13:07:32 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jens Axboe <axboe@...nel.dk>, Sitsofe Wheeler <sitsofe@...oo.com>,
	Borislav Petkov <bp@...en8.de>, Meelis Roos <mroos@...ux.ee>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 2/3 v2.6.39-rc7] block: make disk_block_events()
 properly wait for work cancellation

On Wed, May 18, 2011 at 12:04:51PM +0200, Tejun Heo wrote:
> Yeap, the problem was unblock/check being allowed to be called without
> sleeping context, which isn't used anymore and was broken due to
> cancellation race.  We can just enclose the whole thing inside per-ev
> mutex and everything should be simple and fine.  I'll post patches
> soon.

Oops, spoke too soon.  Converting to mutex creates a circular
dependency.  disk_block_events() acquires ev->lock and waits for
ev->dwork to finish, but ev->dwork also needs to acquire ev->lock to
update event states.  So, no matter how we twist this, we need two
locks or a lock and a completion.  The mutex approach seems simple
enough, so I suggest leaving it like that.

Thanks.

-- 
tejun
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