linux-kernel - Re: fs: WARNING in locks_free_lock

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Date:	Wed, 3 Feb 2016 13:26:21 -0500
From:	Jeff Layton <jlayton@...chiereds.net>
To:	William Dauchy <wdauchy@...il.com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: fs: WARNING in locks_free_lock_context()

On Wed, 3 Feb 2016 19:19:37 +0100
William Dauchy <wdauchy@...il.com> wrote:

> Hello Jeff,
> 
> On Wed, Dec 23, 2015 at 2:54 PM, Jeff Layton <jlayton@...chiereds.net> wrote:
> > Ooh, nice catch...and just in time for Christmas.
> >
> > filp_close does this after the fd has been detached from the file table
> > in __close_fd:
> >
> >         if (likely(!(filp->f_mode & FMODE_PATH))) {
> >                 dnotify_flush(filp, id);
> >                 locks_remove_posix(filp, id);
> >         }
> >         fput(filp);
> >
> > ...and fcntl_setlk does this:
> >
> >         /*
> >          * Attempt to detect a close/fcntl race and recover by
> >          * releasing the lock that was just acquired.
> >          */
> >         /*
> >          * we need that spin_lock here - it prevents reordering between
> >          * update of i_flctx->flc_posix and check for it done in close().
> >          * rcu_read_lock() wouldn't do.
> >          */
> >         spin_lock(&current->files->file_lock);
> >         f = fcheck(fd);
> >         spin_unlock(&current->files->file_lock);
> >         if (!error && f != filp && flock.l_type != F_UNLCK) {
> >                 flock.l_type = F_UNLCK;
> >                 goto again;
> >         }
> >
> > ...so in principle that should keep new locks from racing onto the list
> > just after we call filp_close. Hmm...I'll see if I can reproduce and
> > figure out how this could happen.  
> 
> Just wondering if you had the time to figure out this warning?
> 
> Thanks,

Yes...this commit in mainline fixes it:

commit 7f3697e24dc3820b10f445a4a7d914fc356012d1
Author: Jeff Layton <jeff.layton@...marydata.com>
Date:   Thu Jan 7 16:38:10 2016 -0500

    locks: fix unlock when fcntl_setlk races with a close


...and the patch is applicable to all kernels currently in circulation.
The original bug is very old (from 2005).

-- 
Jeff Layton <jlayton@...chiereds.net>