[PATCH] md: ensure md devices are freed before module is unloaded.

[NeilBrown <neilb@suse.com>]

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Commit: cbd199837750 ("md: Fix unfortunate interaction with evms")
change mddev_put() so that it would not destroy an md device while
->ctime was non-zero.

Unfortunately, we didn't make sure to clear ->ctime when unloading
the module, so it is possible for an md device to remain after
module unload.  An attempt to open such a device will trigger
an invalid memory reference in:
  get_gendisk -> kobj_lookup -> exact_lock -> get_disk

when tring to access disk->fops, which was in the module that has
been removed.

So ensure we clear ->ctime in md_exit(), and explain how that is useful,
as it isn't immediately obvious when looking at the code.

Fixes: cbd199837750 ("md: Fix unfortunate interaction with evms")
Tested-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>
---
 drivers/md/md.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 01175dac0db6..8926fb781cdc 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8980,7 +8980,14 @@ static __exit void md_exit(void)
 
 	for_each_mddev(mddev, tmp) {
 		export_array(mddev);
+		mddev->ctime = 0;
 		mddev->hold_active = 0;
+		/* for_each_mddev() will call mddev_put() at the
+		 * end of each iteration.  As the mddev is now
+		 * fully clear, this will schedule the mddev for destruction
+		 * by a workqueue, and the destroy_workqueue() below
+		 * will wait for that to complete.
+		 */
 	}
 	destroy_workqueue(md_misc_wq);
 	destroy_workqueue(md_wq);
-- 
2.11.0


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