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

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Date:   Tue, 12 Jun 2018 17:57:54 +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 Mon 11-06-18 10:20:53, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jun 11, 2018 at 06:29:20PM +0200, Jan Kara wrote:
> > > Would something like the following work or am I missing the point
> > > entirely?
> > 
> > I was pondering the same solution for a while but I think it won't work.
> > The problem is that e.g. wb_memcg_offline() could have already removed
> > wb from the radix tree but it is still pending in bdi->wb_list
> > (wb_shutdown() has not run yet) and so we'd drop reference we didn't get.
> 
> 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
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.

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