linux-kernel - Re: [PATCH] md: dm-verity: Fix to avoid a deadlock in dm-bufio

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Message-ID: <CAKwO_5GOMN+GL+SO4cBNyhSK_OEbrrDYR=wwEWbgG9S51=tJQg@mail.gmail.com>
Date:	Mon, 4 Mar 2013 13:21:31 -0800
From:	Paul Taysom <taysom@...gle.com>
To:	Paul Menzel <pm.debian@...glemail.com>
Cc:	Paul Taysom <taysom@...omium.org>, agk@...hat.com,
	dm-devel@...hat.com, neilb@...e.de, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Mandeep Singh Baines <msb@...omium.org>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Olof Johansson <olofj@...omium.org>
Subject: Re: [PATCH] md: dm-verity: Fix to avoid a deadlock in dm-bufio

On Mon, Mar 4, 2013 at 9:19 AM, Paul Menzel <pm.debian@...glemail.com> wrote:
>
> Am Montag, den 04.03.2013, 08:45 -0800 schrieb Paul Taysom:
> > Changed the dm-verity prefetching to use a worker thread to avoid
> > a deadlock in dm-bufio.
>
> Do you know of any bug reports about this? Searching for »dm-verity dead
> lock« I found [1], which seems to be the issue, right? If yes, please
> reference it.
>
> > If generic_make_request is called recursively, it queues the I/O
> > request on the current->bio_list without making the I/O request
> > and returns. The routine making the recursive call cannot wait
> > for the I/O to complete.
> >
> > The deadlock occurred when one thread grabbed the bufio_client
> > mutex and waited for an I/O to complete but the I/O was queued
> > on another thread’s current->bio_list and it was waiting to get
> > the mutex held by the first thread.
> >
> > The fix allows only one I/O request from dm-verity to dm-bufio
> > per thread. To do this, the prefetch requests were queued on worker
> > threads.
> >
> > In addition to avoiding the deadlock, this fix made a slight
> > improvement in performance.
> >
> > seconds_kernel_to_login:
> >   with prefetch:    8.43s
> >   without prefetch: 9.2s
> >   worker prefetch:  8.28s
> >
> > Signed-off-by: Paul Taysom <taysom@...omium.org>
> > ---
> >  drivers/md/dm-verity.c | 29 +++++++++++++++++++++++++++--
> >  1 file changed, 27 insertions(+), 2 deletions(-)
>
> Acked-by: Paul Menzel <paulepanter@...rs.sourceforge.net>
>
>
> Thanks,
>
> Paul
>
>
> [1] https://code.google.com/p/chromium-os/issues/detail?id=39148

Yes, I should have included that this fix addresses http://crosbug.com/39148
Thanks,
Paul
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