Re: [RFC PATCH v2 01/10] rv/da: fix monitor start ordering and memory ordering for monitoring flag

[Gabriele Monaco <gmonaco@redhat.com>]

On Tue, 2026-05-12 at 02:24 +0800, wen.yang@linux.dev wrote:
> From: Wen Yang <wen.yang@linux.dev>
> 
> da_monitor_start() set monitoring=1 before calling da_monitor_init_hook(),
> may racing with the sched_switch handler:
> 
>   da_monitor_start()               sched_switch handler
>   -------------------------        ---------------------------------
>   da_mon->monitoring = 1;
>                                    if (da_monitoring(da_mon))  /* true  */
>                                        ha_start_timer_ns(...);
>                                        /* hrtimer->base == NULL, crash */
>   da_monitor_init_hook(da_mon);
>   /* hrtimer_setup() sets base */
> 
> Fix the ordering and pair with release/acquire semantics:
> 
>   da_monitor_init_hook(da_mon);
>   smp_store_release(&da_mon->monitoring, 1);    /* da_monitor_start()  */
>   return smp_load_acquire(&da_mon->monitoring); /* da_monitoring()     */
> 
> On ARM64 a plain STR + LDR does not form a release-acquire pair, so
> the load can observe monitoring=1 while hrtimer->base is still NULL.
> The plain accesses are also data races under KCSAN.
> 
> Use WRITE_ONCE for the monitoring=0 store in da_monitor_reset() to
> cover the reset path.
> 
> Fixes: 792575348ff7 ("rv/include: Add deterministic automata monitor
> definition via C macros")
> Signed-off-by: Wen Yang <wen.yang@linux.dev>

Thanks for the fix!

There are probably more than a few of those bugs since most monitors are
implicitly serialised because their events are serialised..

Looks good to me.

Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>

See minor comments below:

> ---
>  include/rv/da_monitor.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/include/rv/da_monitor.h b/include/rv/da_monitor.h
> index 39765ff6f098..00ded3d5ab3f 100644
> --- a/include/rv/da_monitor.h
> +++ b/include/rv/da_monitor.h
> @@ -82,7 +82,7 @@ static void react(enum states curr_state, enum events event)
>  static inline void da_monitor_reset(struct da_monitor *da_mon)
>  {
>   da_monitor_reset_hook(da_mon);
> - da_mon->monitoring = 0;
> + WRITE_ONCE(da_mon->monitoring, 0);
>   da_mon->curr_state = model_get_initial_state();
>  }
>  
> @@ -95,8 +95,9 @@ static inline void da_monitor_reset(struct da_monitor
> *da_mon)
>  static inline void da_monitor_start(struct da_monitor *da_mon)
>  {
>   da_mon->curr_state = model_get_initial_state();
> - da_mon->monitoring = 1;
>   da_monitor_init_hook(da_mon);
> + /* Pairs with smp_load_acquire in da_monitoring(). */

I wonder if these comment are really adding value, pairing smp_load_acquire /
smp_store_release is the by-the-book usage and everything is here.

But feel free to leave it if you think it's clearer.

Thanks,
Gabriele

> + smp_store_release(&da_mon->monitoring, 1);
>  }
>  
>  /*
> @@ -104,7 +105,8 @@ static inline void da_monitor_start(struct da_monitor
> *da_mon)
>   */
>  static inline bool da_monitoring(struct da_monitor *da_mon)
>  {
> - return da_mon->monitoring;
> + /* Pairs with smp_store_release in da_monitor_start(). */
> + return smp_load_acquire(&da_mon->monitoring);
>  }
>  
>  /*