[RFC] bdrv_flush: only use fast path when in owned AioContext

[Stefan Reiter <s.reiter@proxmox.com>]

Just because we're in a coroutine doesn't imply ownership of the context
of the flushed drive. In such a case use the slow path which explicitly
enters bdrv_flush_co_entry in the correct AioContext.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---

We've experienced some lockups in this codepath when taking snapshots of VMs
with drives that have IO-Threads enabled (we have an async 'savevm'
implementation running from a coroutine).

Currently no reproducer for upstream versions I could find, but in testing this
patch fixes all issues we're seeing and I think the logic checks out.

The fast path pattern is repeated a few times in this file, so if this change
makes sense, it's probably worth evaluating the other occurences as well.

 block/io.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/io.c b/block/io.c
index aba67f66b9..ee7310fa13 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2895,8 +2895,9 @@ int bdrv_flush(BlockDriverState *bs)
         .ret = NOT_DONE,
     };
 
-    if (qemu_in_coroutine()) {
-        /* Fast-path if already in coroutine context */
+    if (qemu_in_coroutine() &&
+        bdrv_get_aio_context(bs) == qemu_get_current_aio_context()) {
+        /* Fast-path if already in coroutine and we own the drive's context */
         bdrv_flush_co_entry(&flush_co);
     } else {
         co = qemu_coroutine_create(bdrv_flush_co_entry, &flush_co);
-- 
2.20.1