block: fix stacked driver stats init and free

If a driver allocates a queue for stacked usage, then it does
not currently get stats allocated. This causes the later init
of, eg, writeback throttling to blow up. Move the init to the
queue allocation instead.

Additionally, allow a NULL callback unregistration. This avoids
having the caller check for that, fixing another oops on
removal of a block device that doesn't have poll stats allocated.

Fixes: 34dbad5d26e2 ("blk-stat: convert to callback-based statistics reporting")
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/block/blk-stat.c b/block/blk-stat.c
index 0d8721a60db9..188b535cf4d6 100644
--- a/block/blk-stat.c
+++ b/block/blk-stat.c
@@ -210,7 +210,8 @@ static void blk_stat_free_callback_rcu(struct rcu_head *head)
 
 void blk_stat_free_callback(struct blk_stat_callback *cb)
 {
-        call_rcu(&cb->rcu, blk_stat_free_callback_rcu);
+        if (cb)
+                call_rcu(&cb->rcu, blk_stat_free_callback_rcu);
 }
 EXPORT_SYMBOL_GPL(blk_stat_free_callback);