md: Avoid waking up a thread after it has been freed.

Two related problems:

1/ some error paths call "md_unregister_thread(mddev->thread)"
   without subsequently clearing ->thread.  A subsequent call
   to mddev_unlock will try to wake the thread, and crash.

2/ Most calls to md_wakeup_thread are protected against the thread
   disappeared either by:
      - holding the ->mutex
      - having an active request, so something else must be keeping
        the array active.
   However mddev_unlock calls md_wakeup_thread after dropping the
   mutex and without any certainty of an active request, so the
   ->thread could theoretically disappear.
   So we need a spinlock to provide some protections.

So change md_unregister_thread to take a pointer to the thread
pointer, and ensure that it always does the required locking, and
clears the pointer properly.

Reported-by: "Moshe Melnikov" <moshe@zadarastorage.com>
Signed-off-by: NeilBrown <neilb@suse.de>
cc: stable@kernel.org

1 files changed, 2 insertions, 3 deletions

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index d7a8468ddeab..0cd9672cf9cb 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -2955,7 +2955,7 @@ static int run(mddev_t *mddev)
 	return 0;
 
 out_free_conf:
-	md_unregister_thread(mddev->thread);
+	md_unregister_thread(&mddev->thread);
 	if (conf->r10bio_pool)
 		mempool_destroy(conf->r10bio_pool);
 	safe_put_page(conf->tmppage);
@@ -2973,8 +2973,7 @@ static int stop(mddev_t *mddev)
 	raise_barrier(conf, 0);
 	lower_barrier(conf);
 
-	md_unregister_thread(mddev->thread);
-	mddev->thread = NULL;
+	md_unregister_thread(&mddev->thread);
 	blk_sync_queue(mddev->queue); /* the unplug fn references 'conf'*/
 	if (conf->r10bio_pool)
 		mempool_destroy(conf->r10bio_pool);