Am 06.02.2024 um 20:06 hat Stefan Hajnoczi geschrieben: > 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 <hre...@redhat.com> > Reviewed-by: Manos Pitsidianakis <manos.pitsidiana...@linaro.org> > Reviewed-by: Hanna Czenczek <hre...@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > --- > qapi/qmp-dispatch.c | 7 ++----- > 1 file changed, 2 insertions(+), 5 deletions(-) > > 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());
Turns out that this one actually causes a regression. [1] This code is ŕun in iohandler_ctx, aio_co_reschedule_self() looks at the new context and compares it with qemu_get_current_aio_context() - and because both are qemu_aio_context, it decides that it has nothing to do. So the command handler coroutine actually still runs in iohandler_ctx now, which is not what we want. We could just revert this patch because it was only meant as a cleanup without a semantic difference. Or aio_co_reschedule_self() could look at qemu_coroutine_self()->ctx instead of using qemu_get_current_aio_context(). That would be a little more indirect, though, and I'm not sure if co->ctx is always up to date. Any opinions on what is the best way to fix this? Kevin [1] https://issues.redhat.com/browse/RHEL-34618
