throttle: acquire the ThrottleGroup lock in bdrv_swap()

bdrv_swap() touches the fields of a BlockDriverState that are
protected by the ThrottleGroup lock. Although those fields end up in
their original place, they are temporarily swapped in the process,
so there's a chance that an operation on a member of the same group
happening on a different thread can try to use them.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: d92dc40d7c4f1fc5cda5cbbf4ffb7a4670b79d17.1433779731.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 30 insertions, 1 deletions

diff --git a/block/throttle-groups.c b/block/throttle-groups.c
index da8c70c..efc462f 100644
--- a/block/throttle-groups.c
+++ b/block/throttle-groups.c
@@ -33,7 +33,8 @@
  * its own locking.
  *
  * This locking is however handled internally in this file, so it's
- * transparent to outside users.
+ * mostly transparent to outside users (but see the documentation in
+ * throttle_groups_lock()).
  *
  * The whole ThrottleGroup structure is private and invisible to
  * outside users, that only use it through its ThrottleState.
@@ -459,6 +460,34 @@ void throttle_group_unregister_bs(BlockDriverState *bs)
     bs->throttle_state = NULL;
 }
 
+/* Acquire the lock of this throttling group.
+ *
+ * You won't normally need to use this. None of the functions from the
+ * ThrottleGroup API require you to acquire the lock since all of them
+ * deal with it internally.
+ *
+ * This should only be used in exceptional cases when you want to
+ * access the protected fields of a BlockDriverState directly
+ * (e.g. bdrv_swap()).
+ *
+ * @bs: a BlockDriverState that is member of the group
+ */
+void throttle_group_lock(BlockDriverState *bs)
+{
+    ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
+    qemu_mutex_lock(&tg->lock);
+}
+
+/* Release the lock of this throttling group.
+ *
+ * See the comments in throttle_group_lock().
+ */
+void throttle_group_unlock(BlockDriverState *bs)
+{
+    ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
+    qemu_mutex_unlock(&tg->lock);
+}
+
 static void throttle_groups_init(void)
 {
     qemu_mutex_init(&throttle_groups_lock);