linux-kernel - Re: [PATCH v4] slow-work: add (module*)work->ops->owner to fix races with module clients

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Date:	Tue, 30 Jun 2009 13:30:01 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	steve@...gwyn.com
Cc:	Steven Whitehouse <swhiteho@...hat.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	linux-kernel@...r.kernel.org, dhowells@...hat.com
Subject: Re: [PATCH v4] slow-work: add (module*)work->ops->owner to fix
	races with module clients

On Tue, Jun 30, 2009 at 10:18:32AM +0100, steve@...gwyn.com wrote:
> Hi,
> 
> On Tue, Jun 30, 2009 at 12:07:15PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Jun 30, 2009 at 09:43:03AM +0100, Steven Whitehouse wrote:
> > > Hi,
> > > 
> > > I'm happy to ACK this, but the race doesn't exist in GFS2's case because
> > > we wait for all work related to each GFS2 fs at umount time and the
> > > module unload cannot happen until all GFS2 fs are umounted,
> > > 
> > > Steve.
> > 
> > I wonder whether the following holds:
> > 
> > static void gfs2_recover_put_ref(struct slow_work *work)
> > {
> >         struct gfs2_jdesc *jd = container_of(work, struct gfs2_jdesc, jd_work);
> >         clear_bit(JDF_RECOVERY, &jd->jd_flags);
> >         smp_mb__after_clear_bit();
> >         wake_up_bit(&jd->jd_flags, JDF_RECOVERY);
> > 
> > <- umount can complete here?
> > 
> > }
> > 
> > 
> > If yes, .text of the module could go away between the point marked by <-
> > and return from gfs2_recover_put_ref.
> > 
> >
> Well in theory, yes. In reality I don't think it could ever happen

Right. IIUC, that's all Gregory's patch is trying to address: a
theoretical race condition.

-- 
MST
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