[PATCH for-8.2 1/4] block: Fix bdrv_graph_wrlock() call in blk_remove_bs()

[Kevin Wolf <kwolf@redhat.com>]

While not all callers of blk_remove_bs() are correct in this respect,
the assumption in the function is that callers hold the AioContext lock
of the BlockBackend (this is required by the drain calls in it).

In order to avoid deadlock in the nested event loop, bdrv_graph_wrlock()
has then to be called with the root BlockDriverState as its parameter
instead of NULL, so that this AioContext lock is temporarily dropped.

Fixes: https://issues.redhat.com/browse/RHEL-1761
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/block-backend.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 4053134781..f6f05ead28 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -882,6 +882,8 @@ BlockBackend *blk_by_public(BlockBackendPublic *public)
 
 /*
  * Disassociates the currently associated BlockDriverState from @blk.
+ *
+ * The caller must hold the AioContext lock for the BlockBackend.
  */
 void blk_remove_bs(BlockBackend *blk)
 {
@@ -916,7 +918,7 @@ void blk_remove_bs(BlockBackend *blk)
     root = blk->root;
     blk->root = NULL;
 
-    bdrv_graph_wrlock(NULL);
+    bdrv_graph_wrlock(root->bs);
     bdrv_root_unref_child(root);
     bdrv_graph_wrunlock();
 }
-- 
2.41.0