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[Xen-devel] [PATCH AUTOSEL 4.9 12/14] xen-blkback: prevent premature module unload

From: Paul Durrant <pdurrant@xxxxxxxxxx>

[ Upstream commit fa2ac657f9783f0891b2935490afe9a7fd29d3fa ]

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>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
 drivers/block/xen-blkback/xenbus.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/block/xen-blkback/xenbus.c 
index ad736d7de8383..1d1f866579679 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -178,6 +178,15 @@ 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 freed back to the cache.
+        */
+       __module_get(THIS_MODULE);
        INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
        return blkif;
@@ -322,6 +331,7 @@ 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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