linux-kernel - Re: possible deadlock in generic_file_write

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Date:   Thu, 7 Dec 2017 18:07:48 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Byungchul Park <byungchul.park@....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 Wed, Dec 6, 2017 at 6:05 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.

I've enabled CONFIG_BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK. It
should have the same effect, right?