linux-kernel - Re: possible deadlock in generic_file_write

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Date:   Tue, 5 Dec 2017 10:19:07 +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 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?

Thanks