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[Xen-devel] [PATCH] 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 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()

Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
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 
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);
+       /*
+        * 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.
+        */
+       __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)

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