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

-----Original Message-----
From: Roger Pau Monné <redacted>
Sent: 11 December 2019 11:29
To: Durrant, Paul <redacted>
Cc: xen-devel@lists.xenproject.org; linux-block@vger.kernel.org; linux-
kernel@vger.kernel.org; Konrad Rzeszutek Wilk [off-list ref];
Jens Axboe [off-list ref]
Subject: Re: [PATCH] xen-blkback: prevent premature module unload

On Tue, Dec 10, 2019 at 02:53:05PM +0000, Paul Durrant wrote:

quoted

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 <redacted>

Reviewed-by: Roger Pau Monné <redacted>

One nit below.

quoted

---
Cc: Konrad Rzeszutek Wilk <redacted>
Cc: "Roger Pau Monné" <redacted>
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

quoted

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)

quoted

 	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

quoted

+	 */
+	__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