md: ensure md devices are freed before module is unloaded.

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>
Signed-off-by: Shaohua Li <shli@fb.com>

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 01175dac0db6361f04cd1bcad910a59202cc276d..13020e5c8cc0b7eedcf909790b7df44cffc977cb 100644 (file)
--- 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);