Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 12 Jun 2018 17:57:54 +0200
From:   Jan Kara <jack@suse.cz>
To:     Tejun Heo <tj@kernel.org>
Cc:     Jan Kara <jack@suse.cz>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Dmitry Vyukov <dvyukov@google.com>,
        Jens Axboe <axboe@kernel.dk>,
        syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <david@fromorbit.com>,
        linux-block@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn()
Message-ID: <20180612155754.x5k2yndh5t6wlmpy@quack2.suse.cz>
References: <CACT4Y+Y7Mj1JngLst1aRHDhURXQMn-eTjyPFjDdGAT0ZV-dHrw@mail.gmail.com>
 <CACT4Y+bBfwpcP2h0URpqwiNMQ5SFJdPDHThUu2xetmrxgC+3BQ@mail.gmail.com>
 <CACT4Y+bHDWDapwOonO1EpR6TiRa=qf9nSDtHArq75yCGuHf=gg@mail.gmail.com>
 <CACT4Y+Z4i=bD49cb4pJ5zOMxVu675NT8wCNk+c2n2XJVOXG2bg@mail.gmail.com>
 <2b437c6f-3e10-3d83-bdf3-82075d3eaa1a@i-love.sakura.ne.jp>
 <3cf4b0e3-31b6-8cdc-7c1e-15ba575a7879@i-love.sakura.ne.jp>
 <20180611091248.2i6nt27h5mxrodm2@quack2.suse.cz>
 <20180611160131.GQ1351649@devbig577.frc2.facebook.com>
 <20180611162920.mwapvuqotvhkntt3@quack2.suse.cz>
 <20180611172053.GR1351649@devbig577.frc2.facebook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180611172053.GR1351649@devbig577.frc2.facebook.com>
User-Agent: NeoMutt/20170421 (1.8.2)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@suse.com>
SUSE Labs, CR