blockdev: mirror: avoid potential deadlock when using iothread

The bdrv_getlength() function is a generated co-wrapper and uses
AIO_WAIT_WHILE() to wait for the spawned coroutine. AIO_WAIT_WHILE()
expects the lock to be acquired exactly once.

Fix a case where it may be acquired twice. This can happen when the
source node is explicitly specified as the @Replaces parameter or if the
source node is a filter node.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20231019131936.414246-4-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -2968,6 +2968,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
 
     if (replaces) {
         BlockDriverState *to_replace_bs;
+        AioContext *aio_context;
         AioContext *replace_aio_context;
         int64_t bs_size, replace_size;
 
@@ -2982,10 +2983,19 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
             return;
         }
 
+        aio_context = bdrv_get_aio_context(bs);
         replace_aio_context = bdrv_get_aio_context(to_replace_bs);
-        aio_context_acquire(replace_aio_context);
+        /*
+         * bdrv_getlength() is a co-wrapper and uses AIO_WAIT_WHILE. Be sure not
+         * to acquire the same AioContext twice.
+         */
+        if (replace_aio_context != aio_context) {
+            aio_context_acquire(replace_aio_context);
+        }
         replace_size = bdrv_getlength(to_replace_bs);
-        aio_context_release(replace_aio_context);
+        if (replace_aio_context != aio_context) {
+            aio_context_release(replace_aio_context);
+        }
 
         if (replace_size < 0) {
             error_setg_errno(errp, -replace_size,