Re: [PATCH] rcu: Unify force quiescent state

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

On 10/16/25 09:34, Akihiko Odaki wrote:
> Borrow the concept of force quiescent state from Linux to ensure readers
> remain fast during normal operation and to avoid stalls.
> 
> Background
> ==========
> 
> The previous implementation had four steps to begin reclamation.
> 
> 1. call_rcu_thread() would wait for the first callback.
> 
> 2. call_rcu_thread() would periodically poll until a decent number of
>    callbacks piled up or it timed out.
> 
> 3. synchronize_rcu() would statr a grace period (GP).
> 
> 4. wait_for_readers() would wait for the GP to end. It would also
>    trigger the force_rcu notifier to break busy loops in a read-side
>    critical section if drain_call_rcu() had been called.
> 
> Problem
> =======
> 
> The separation of waiting logic across these steps led to suboptimal
> behavior:
> 
> The GP was delayed until call_rcu_thread() stops polling.
> 
> force_rcu was not consistently triggered when call_rcu_thread() detected
> a high number of pending callbacks or a timeout. This inconsistency
> sometimes led to stalls, as reported in a virtio-gpu issue where memory
> unmapping was blocked[1].
> 
> wait_for_readers() imposed unnecessary overhead in non-urgent cases by
> unconditionally executing qatomic_set(&index->waiting, true) and
> qemu_event_reset(&rcu_gp_event), which are necessary only for expedited
> synchronization.
> 
> Solution
> ========
> 
> Move the polling in call_rcu_thread() to wait_for_readers() to prevent
> the delay of the GP. Additionally, reorganize wait_for_readers() to
> distinguish between two states:
> 
> Normal State: it relies exclusively on periodic polling to detect
> the end of the GP and maintains the read-side fast path.
> 
> Force Quiescent State: Whenever expediting synchronization, it always
> triggers force_rcu and executes both qatomic_set(&index->waiting, true)
> and qemu_event_reset(&rcu_gp_event). This avoids stalls while confining
> the read-side overhead to this state.
> 
> This unified approach, inspired by the Linux RCU, ensures consistent and
> efficient RCU grace period handling and confirms resolution of the
> virtio-gpu issue.
> 
> [1] https://lore.kernel.org/qemu-devel/20251014111234.3190346-9-alex.bennee@linaro.org/
> 
> Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> ---
>  util/rcu.c | 79 ++++++++++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 51 insertions(+), 28 deletions(-)

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # drm native
contexts + venus


-- 
Best regards,
Dmitry