Re: [PATCH] rcu: Unify force quiescent state

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 10/16/25 09:34, Akihiko Odaki wrote:
> -        /* Wait for one thread to report a quiescent state and try again.
> +        /*
> +         * Sleep for a while and try again.
>           * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
>           * wait too much time.
>           *
> @@ -133,7 +150,20 @@ static void wait_for_readers(void)
>           * rcu_registry_lock is released.
>           */
>          qemu_mutex_unlock(&rcu_registry_lock);
> -        qemu_event_wait(&rcu_gp_event);
> +
> +        if (forced) {
> +            qemu_event_wait(&rcu_gp_event);
> +
> +            /*
> +             * We want to be notified of changes made to rcu_gp_ongoing
> +             * while we walk the list.
> +             */
> +            qemu_event_reset(&rcu_gp_event);
> +        } else {
> +            g_usleep(10000);
> +            sleeps++;

Thanks a lot for this RCU improvement. It indeed removes the hard stalls
with unmapping of virtio-gpu blobs.

Am I understanding correctly that potentially we will be hitting this
g_usleep(10000) and stall virtio-gpu for the first ~10ms? I.e. the
MemoryRegion patches from Alex [1] are still needed to avoid stalls
entirely.

[1]
https://lore.kernel.org/qemu-devel/20251014111234.3190346-6-alex.bennee@linaro.org/

-- 
Best regards,
Dmitry