RE: [PATCH] xen-blkback: prevent premature module unload

[Durrant, Paul]

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Sent: 11 December 2019 13:55
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; Juergen Gross <jgross@xxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
> Jens Axboe <axboe@xxxxxxxxx>
> Subject: Re: [PATCH] xen-blkback: prevent premature module unload
> 
> On Wed, Dec 11, 2019 at 01:27:42PM +0000, Durrant, Paul wrote:
> > > -----Original Message-----
> > > From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > Sent: 11 December 2019 11:29
> > > To: Durrant, Paul <pdurrant@xxxxxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx;
> linux-
> > > kernel@xxxxxxxxxxxxxxx; Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx>;
> > > Jens Axboe <axboe@xxxxxxxxx>
> > > Subject: Re: [PATCH] xen-blkback: prevent premature module unload
> > >
> > > On Tue, Dec 10, 2019 at 02:53:05PM +0000, Paul Durrant wrote:
> > > > Objects allocated by xen_blkif_alloc come from the 'blkif_cache'
> kmem
> > > > cache. This cache is destoyed when xen-blkif is unloaded so it is
> > > > necessary to wait for the deferred free routine used for such
> objects to
> > > > complete. This necessity was missed in commit 14855954f636 "xen-
> blkback:
> > > > allow module to be cleanly unloaded". This patch fixes the problem
> by
> > > > taking/releasing extra module references in xen_blkif_alloc/free()
> > > > respectively.
> > > >
> > > > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > >
> > > Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > >
> > > One nit below.
> > >
> > > > ---
> > > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > > Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
> > > > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > > > ---
> > > >  drivers/block/xen-blkback/xenbus.c | 10 ++++++++++
> > > >  1 file changed, 10 insertions(+)
> > > >
> > > > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-
> > > blkback/xenbus.c
> > > > index e8c5c54e1d26..59d576d27ca7 100644
> > > > --- a/drivers/block/xen-blkback/xenbus.c
> > > > +++ b/drivers/block/xen-blkback/xenbus.c
> > > > @@ -171,6 +171,15 @@ static struct xen_blkif
> *xen_blkif_alloc(domid_t
> > > domid)
> > > >  	blkif->domid = domid;
> > > >  	atomic_set(&blkif->refcnt, 1);
> > > >  	init_completion(&blkif->drain_complete);
> > > > +
> > > > +	/*
> > > > +	 * Because freeing back to the cache may be deferred, it is
> not
> > > > +	 * safe to unload the module (and hence destroy the cache)
> until
> > > > +	 * this has completed. To prevent premature unloading, take an
> > > > +	 * extra module reference here and release only when the
> object
> > > > +	 * has been free back to the cache.
> > >                     ^ freed
> >
> > Oh yes. Can this be done on commit, or would you like me to send a v2?
> 
> Adjusting on commit would be fine for me, but it's up to Juergen since
> he is the one that will pick this up. IIRC the module unload patches
> didn't go through the block subsystem.

True. I forgot manually add Juergen cc list.

  Paul

> 
> Thanks, Roger.