linux-kernel - Re: possible deadlock in generic_file_write

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Message-ID: <20171205094150.GA6076@quack2.suse.cz>
Date:   Tue, 5 Dec 2017 10:41:50 +0100
From:   Jan Kara <jack@...e.cz>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Jan Kara <jack@...e.cz>,
        syzbot 
        <bot+045a1f65bdea780940bf0f795a292f4cd0b773d1@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, hannes@...xchg.org, jlayton@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mgorman@...hsingularity.net, npiggin@...il.com, rgoldwyn@...e.com,
        syzkaller-bugs@...glegroups.com, peterz@...radead.org,
        kernel-team@....com
Subject: Re: possible deadlock in generic_file_write_iter (2)


Hello Byungchul,

On Tue 05-12-17 13:58:09, Byungchul Park wrote:
> On 12/4/2017 5:33 PM, Jan Kara wrote:
> >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:

OK, thanks for the tip.

> 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.

But this is something I don't get: There aren't sb_writers anywhere near
put_ucounts(). So why the heck did lockdep think that sb_writers are
acquired by put_ucounts()?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR