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monitor: use aio_co_reschedule_self()

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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 <hreitz@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240206190610.107963-6-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2024-02-06 14:06:10 -05:00 committed by Kevin Wolf
parent 8802f4f4e3
commit 1f25c172f8
1 changed files with 2 additions and 5 deletions
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7
qapi/qmp-dispatch.c
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@ -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 * executing the command handler so that it can make progress if it
* involves an AIO_WAIT_WHILE(). * involves an AIO_WAIT_WHILE().
*/ */
aio_co_schedule(qemu_get_aio_context(), qemu_coroutine_self()); aio_co_reschedule_self(qemu_get_aio_context());
qemu_coroutine_yield();
} }
monitor_set_cur(qemu_coroutine_self(), cur_mon); 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 * Move back to iohandler_ctx so that nested event loops for
* qemu_aio_context don't start new monitor commands. * qemu_aio_context don't start new monitor commands.
*/ */
aio_co_schedule(iohandler_get_aio_context(), aio_co_reschedule_self(iohandler_get_aio_context());
qemu_coroutine_self());
qemu_coroutine_yield();
} }
} else { } else {
/* /*