block-backend: Update ctx immediately after root

blk_get_aio_context() asserts that blk->ctx is always equal to the root
BDS's context (if there is a root BDS).  Therefore,
blk_do_set_aio_context() must update blk->ctx immediately after the root
BDS's context has changed.

Without this patch, the next patch would break iotest 238, because
bdrv_drained_begin() (called by blk_do_set_aio_context()) may then
invoke bdrv_child_get_parent_aio_context() on the root child, i.e.
blk_get_aio_context().  However, by this point, blk->ctx would not have
been updated and thus differ from the root node's context.  This patch
fixes that.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20221107151321.211175-3-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index ed2f4b6..b48c91f 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2158,6 +2158,11 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
                 return ret;
             }
         }
+        /*
+         * Make blk->ctx consistent with the root node before we invoke any
+         * other operations like drain that might inquire blk->ctx
+         */
+        blk->ctx = new_context;
         if (tgm->throttle_state) {
             bdrv_drained_begin(bs);
             throttle_group_detach_aio_context(tgm);
@@ -2166,9 +2171,10 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
         }
 
         bdrv_unref(bs);
+    } else {
+        blk->ctx = new_context;
     }
 
-    blk->ctx = new_context;
     return 0;
 }