Re: [PATCH] bdi: Fix another oops in wb_workfn()

Jan Kara
2018-06-18T12:27:29Z

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@suse.com>
SUSE Labs, CR