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

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Date:   Fri, 15 Jun 2018 14:06:20 +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 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.

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

View attachment "0001-bdi-Fix-another-oops-in-wb_workfn.patch" of type "text/x-patch" (3886 bytes)