linux-kernel - Re: possible deadlock in generic_file_write

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Date:   Wed, 6 Dec 2017 14:05:47 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Jan Kara <jack@...e.cz>,
        syzbot 
        <bot+045a1f65bdea780940bf0f795a292f4cd0b773d1@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, jlayton@...hat.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>, npiggin@...il.com,
        rgoldwyn@...e.com, syzkaller-bugs@...glegroups.com,
        Peter Zijlstra <peterz@...radead.org>, kernel-team@....com
Subject: Re: possible deadlock in generic_file_write_iter (2)

On Tue, Dec 05, 2017 at 10:19:07AM +0100, Dmitry Vyukov wrote:
> On Tue, Dec 5, 2017 at 5:58 AM, Byungchul Park <byungchul.park@....com> wrote:
> > On 12/4/2017 5:33 PM, Jan Kara wrote:
> >>
> >> Hello,
> >>
> >> adding Peter and Byungchul to CC since the lockdep report just looks
> >> strange and cross-release seems to be involved. Guys, how did #5 get into
> >> the lock chain and what does put_ucounts() have to do with sb_writers
> >> there? Thanks!
> >
> >
> > Hello Jan,
> >
> > In order to get full stack of #5, we have to pass a boot param,
> > "crossrelease_fullstack", to the kernel. Now that it only informs
> > put_ucounts() in the call trace, it's hard to find out what exactly
> > happened at that time, but I can tell #5 shows:
> >
> > When acquire(sb_writers) in put_ucounts(), it was on the way to
> > complete((completion)&req.done) of wait_for_completion() in
> > devtmpfs_create_node().
> >
> > If acquire(sb_writers) in put_ucounts() is stuck, then
> > wait_for_completion() in devtmpfs_create_node() would be also
> > stuck, since complete() being in the context of acquire(sb_writers)
> > cannot be called.
> >
> > This is why cross-release added the lock chain.
> 
> Hi,
> 
> What is cross-release? Is it something new? Should we always enable
> crossrelease_fullstack during testing?

Hello Dmitry,

Yes, it's new one making lockdep track wait_for_completion() as well.

And we should enable crossrelease_fullstack if you don't care system
slowdown but testing.

--
Thanks,
Byungchul