linux-kernel - Re: [PATCH] bdi: Fix another oops in wb

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Date:   Mon, 18 Jun 2018 14:27:29 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jan Kara <jack@...e.cz>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+4a7438e774b21ddd8eca@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        linux-block@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn()

On Fri 15-06-18 14:06:20, Jan Kara wrote:
> On Wed 13-06-18 07:33:15, Tejun Heo wrote:
> > Hello, Jan.
> > 
> > On Tue, Jun 12, 2018 at 05:57:54PM +0200, Jan Kara wrote:
> > > > Yeah, right, so the root cause is that we're walking the wb_list while
> > > > holding lock and expecting the object to stay there even after lock is
> > > > released.  Hmm... we can use a mutex to synchronize the two
> > > > destruction paths.  It's not like they're hot paths anyway.
> > > 
> > > Hmm, do you mean like having a per-bdi or even a global mutex that would
> > > protect whole wb_shutdown()? Yes, that should work and we could get rid of
> > > WB_shutting_down bit as well with that. Just it seems a bit strange to
> > 
> > Yeap.
> > 
> > > introduce a mutex only to synchronize these two shutdown paths - usually
> > > locks protect data structures and in this case we have cgwb_lock for
> > > that so it looks like a duplication from a first look.
> > 
> > Yeah, I feel a bit reluctant too but I think that's the right thing to
> > do here.  This is an inherently weird case where there are two ways
> > that an object can go away with the immediate drain requirement from
> > one side.  It's not a hot path and the dumber the synchronization the
> > better, right?
> 
> Yeah, fair enough. Something like attached patch? It is indeed considerably
> simpler than fixing synchronization using WB_shutting_down. This one even
> got some testing using scsi_debug, I want to do more testing next week with
> more cgroup writeback included.

OK, the test has passed some beating with cgroup writeback running. I'll do
official posting shortly.

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