[PULL,05/16] monitor: use aio_co_reschedule_self()

Commit Message

Kevin Wolf Feb. 7, 2024, 9:55 p.m. UTC

From: Stefan Hajnoczi <[e-mail]>

The aio_co_reschedule_self() API is designed to avoid the race
condition between scheduling the coroutine in another AioContext and
yielding.

The QMP dispatch code uses the open-coded version that appears
susceptible to the race condition at first glance:

  aio_co_schedule(qemu_get_aio_context(), qemu_coroutine_self());
  qemu_coroutine_yield();

The code is actually safe because the iohandler and qemu_aio_context
AioContext run under the Big QEMU Lock. Nevertheless, set a good example
and use aio_co_reschedule_self() so it's obvious that there is no race.

Suggested-by: Hanna Reitz <[e-mail]>
Reviewed-by: Manos Pitsidianakis <[e-mail]>
Reviewed-by: Hanna Czenczek <[e-mail]>
Signed-off-by: Stefan Hajnoczi <[e-mail]>
Message-ID: <[e-mail]>
Reviewed-by: Kevin Wolf <[e-mail]>
Acked-by: Markus Armbruster <[e-mail]>
Reviewed-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Kevin Wolf <[e-mail]>
---
 qapi/qmp-dispatch.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

Patch

diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
index 176b549473..f3488afeef 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -212,8 +212,7 @@  QDict *coroutine_mixed_fn qmp_dispatch(const QmpCommandList *cmds, QObject *requ
              * executing the command handler so that it can make progress if it
              * involves an AIO_WAIT_WHILE().
              */
-            aio_co_schedule(qemu_get_aio_context(), qemu_coroutine_self());
-            qemu_coroutine_yield();
+            aio_co_reschedule_self(qemu_get_aio_context());
         }
 
         monitor_set_cur(qemu_coroutine_self(), cur_mon);
@@ -227,9 +226,7 @@  QDict *coroutine_mixed_fn qmp_dispatch(const QmpCommandList *cmds, QObject *requ
              * Move back to iohandler_ctx so that nested event loops for
              * qemu_aio_context don't start new monitor commands.
              */
-            aio_co_schedule(iohandler_get_aio_context(),
-                            qemu_coroutine_self());
-            qemu_coroutine_yield();
+            aio_co_reschedule_self(iohandler_get_aio_context());
         }
     } else {
        /*