xen-blkback: prevent premature module unload

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 14855954 "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>
Signed-off-by: Juergen Gross <jgross@suse.com>

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 freed back to the cache.
+	 */
+	__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)