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

["Durrant, Paul" <pdurrant@amazon.com>]

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@citrix.com>
> Sent: 11 December 2019 11:29
> To: Durrant, Paul <pdurrant@amazon.com>
> Cc: xen-devel@lists.xenproject.org; linux-block@vger.kernel.org; linux-
> kernel@vger.kernel.org; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>;
> Jens Axboe <axboe@kernel.dk>
> 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@amazon.com>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
> 
> One nit below.
> 
> > ---
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> > Cc: Jens Axboe <axboe@kernel.dk>
> > ---
> >  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?

  Paul

> > +	 */
> > +	__module_get(THIS_MODULE);
> >  	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
> >
> >  	return blkif;
> > @@ -320,6 +329,7 @@ static void xen_blkif_free(struct xen_blkif *blkif)
> >
> >  	/* Make sure everything is drained before shutting down */
> >  	kmem_cache_free(xen_blkif_cachep, blkif);
> > +	module_put(THIS_MODULE);
> >  }
> >
> >  int __init xen_blkif_interface_init(void)
> > --
> > 2.20.1
> >