linux-kernel - Re: [dm-devel] block: flush queued bios when the process blocks

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Date:	Tue, 6 Oct 2015 14:17:03 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Mike Snitzer <snitzer@...hat.com>
cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	"Alasdair G. Kergon" <agk@...hat.com>
Subject: Re: [dm-devel] block: flush queued bios when the process blocks



On Mon, 5 Oct 2015, Mike Snitzer wrote:

> FYI, I've put rebased versions of your 2 patches in my wip branch, see:
> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip

I found a bug in the first patch 
(http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=2e90df2e9cf482f45be4230152535fdab525fbd8)

There is this piece of code:

	spin_lock(&bs->rescue_lock);
	bio_list_add(&bs->rescue_list, bio);
	spin_unlock(&bs->rescue_lock);
	queue_work(bs->rescue_workqueue, &bs->rescue_work);

It is possible that after spin_unlock and before queue_work the bio is 
finished by previous workqueue invocation. When the bio is finished, it is 
possible that the block device is unloaded and queue_work accesses freed 
memory.

Change the code so that queue_work is executed inside the spinlock:
	spin_lock(&bs->rescue_lock);
	bio_list_add(&bs->rescue_list, bio);
	queue_work(bs->rescue_workqueue, &bs->rescue_work);
	spin_unlock(&bs->rescue_lock);

Mikulas
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