From: Kevin Wolf <kwolf@redhat.com>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-devel@nongnu.org, Michael Roth <michael.roth@amd.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Markus Armbruster <armbru@redhat.com>,
Hanna Reitz <hreitz@redhat.com>,
qemu-block@nongnu.org,
Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Subject: Re: [PATCH v2 5/5] monitor: use aio_co_reschedule_self()
Date: Fri, 3 May 2024 19:33:17 +0200 [thread overview]
Message-ID: <ZjUf3TkiBO1N264C@redhat.com> (raw)
In-Reply-To: <20240206190610.107963-6-stefanha@redhat.com>
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?
Kevin
[1] https://issues.redhat.com/browse/RHEL-34618
next prev parent reply other threads:[~2024-05-03 17:34 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-06 19:06 [PATCH v2 0/5] virtio-blk: iothread-vq-mapping cleanups Stefan Hajnoczi
2024-02-06 19:06 ` [PATCH v2 1/5] virtio-blk: enforce iothread-vq-mapping validation Stefan Hajnoczi
2024-02-06 19:06 ` [PATCH v2 2/5] virtio-blk: clarify that there is at least 1 virtqueue Stefan Hajnoczi
2024-02-06 19:06 ` [PATCH v2 3/5] virtio-blk: add vq_rq[] bounds check in virtio_blk_dma_restart_cb() Stefan Hajnoczi
2024-02-06 19:06 ` [PATCH v2 4/5] virtio-blk: declare VirtIOBlock::rq with a type Stefan Hajnoczi
2024-02-06 19:06 ` [PATCH v2 5/5] monitor: use aio_co_reschedule_self() Stefan Hajnoczi
2024-05-03 17:33 ` Kevin Wolf [this message]
2024-05-06 18:09 ` Stefan Hajnoczi
2024-02-06 19:08 ` [PATCH v2 0/5] virtio-blk: iothread-vq-mapping cleanups Michael S. Tsirkin
2024-02-07 13:45 ` Kevin Wolf
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