Re: [PATCH v2 5/5] monitor: use aio_co_reschedule_self()

[Stefan Hajnoczi]

On Fri, May 03, 2024 at 07:33:17PM +0200, Kevin Wolf wrote:
> 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 <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>
> > ---
> >  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?

If the commit is reverted then similar bugs may be introduced again in
the future. The qemu_get_current_aio_context() API is unaware of
iohandler_ctx and this can lead to unexpected results.

I will send patches to revert the commit and add doc comments explaining
iohandler_ctx's special behavior. This will reduce, but not eliminate,
the risk of future bugs.

Modifying aio_co_reschedule_self() might be better long-term fix, but
I'm afraid it will create more bugs because it will expose the subtle
distinction between the current coroutine AioContext and non-coroutine
AioContext in new places. I think the root cause is that iohandler_ctx
isn't a full-fledged AioContext with its own event loop. iohandler_ctx
is a special superset of qemu_aio_context that the main loop monitors.

Stefan

> 
> Kevin
> 
> [1] https://issues.redhat.com/browse/RHEL-34618
>

signature.asc
Description: PGP signature