graph-lock: Unlock the AioContext while polling

If the caller keeps the AioContext lock for a block node in an iothread,
polling in bdrv_graph_wrlock() deadlocks if the condition isn't
fulfilled immediately.

Now that all callers make sure to actually have the AioContext locked
when they call bdrv_replace_child_noperm() like they should, we can
change bdrv_graph_wrlock() to take a BlockDriverState whose AioContext
lock the caller holds (NULL if it doesn't) and unlock it temporarily
while polling.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-11-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/block.c b/block.c
index dc691b9832..0cd1a12344 100644
--- a/block.c
+++ b/block.c
@@ -2854,7 +2854,7 @@ uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
  * Replaces the node that a BdrvChild points to without updating permissions.
  *
  * If @new_bs is non-NULL, the parent of @child must already be drained through
- * @child.
+ * @child and the caller must hold the AioContext lock for @new_bs.
  */
 static void bdrv_replace_child_noperm(BdrvChild *child,
                                       BlockDriverState *new_bs)
@@ -2893,7 +2893,7 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
     }
 
     /* TODO Pull this up into the callers to avoid polling here */
-    bdrv_graph_wrlock();
+    bdrv_graph_wrlock(new_bs);
     if (old_bs) {
         if (child->klass->detach) {
             child->klass->detach(child);