blockjob: Fix AioContext locking in block_job_add_bdrv()

bdrv_root_attach_child() requires callers to hold the AioContext lock
for child_bs. Take it in block_job_add_bdrv() before calling the
function.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-10-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockjob.c

@@ -230,20 +230,27 @@ int block_job_add_bdrv(BlockJob *job, const char *name, BlockDriverState *bs,
                        uint64_t perm, uint64_t shared_perm, Error **errp)
 {
     BdrvChild *c;
+    AioContext *ctx = bdrv_get_aio_context(bs);
     bool need_context_ops;
     GLOBAL_STATE_CODE();
 
     bdrv_ref(bs);
 
-    need_context_ops = bdrv_get_aio_context(bs) != job->job.aio_context;
+    need_context_ops = ctx != job->job.aio_context;
 
-    if (need_context_ops && job->job.aio_context != qemu_get_aio_context()) {
-        aio_context_release(job->job.aio_context);
+    if (need_context_ops) {
+        if (job->job.aio_context != qemu_get_aio_context()) {
+            aio_context_release(job->job.aio_context);
+        }
+        aio_context_acquire(ctx);
     }
     c = bdrv_root_attach_child(bs, name, &child_job, 0, perm, shared_perm, job,
                                errp);
-    if (need_context_ops && job->job.aio_context != qemu_get_aio_context()) {
-        aio_context_acquire(job->job.aio_context);
+    if (need_context_ops) {
+        aio_context_release(ctx);
+        if (job->job.aio_context != qemu_get_aio_context()) {
+            aio_context_acquire(job->job.aio_context);
+        }
     }
     if (c == NULL) {
         return -EPERM;