Re: [PATCH v2 3/8] xen/events: avoid handling the same event on two cpus at the same time

[Julien Grall <julien@xxxxxxx>]

Hi Juergen,

On 11/02/2021 10:16, Juergen Gross wrote:
> When changing the cpu affinity of an event it can happen today that
> (with some unlucky timing) the same event will be handled on the old
> and the new cpu at the same time.
>
> Avoid that by adding an "event active" flag to the per-event data and
> call the handler only if this flag isn't set.
>
> Reported-by: Julien Grall <julien@xxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> V2:
> - new patch
> ---
>   drivers/xen/events/events_base.c | 19 +++++++++++++++----
>   1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
> index e157e7506830..f7e22330dcef 100644
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -102,6 +102,7 @@ struct irq_info {
>   #define EVT_MASK_REASON_EXPLICIT      0x01
>   #define EVT_MASK_REASON_TEMPORARY     0x02
>   #define EVT_MASK_REASON_EOI_PENDING   0x04
> +       u8 is_active;           /* Is event just being handled? */
>         unsigned irq;
>         evtchn_port_t evtchn;   /* event channel */
>         unsigned short cpu;     /* cpu bound */
> @@ -622,6 +623,7 @@ static void xen_irq_lateeoi_locked(struct irq_info *info, 
> bool spurious)
>         }
>   
>   	info->eoi_time = 0;
> +       smp_store_release(&info->is_active, 0);
>         do_unmask(info, EVT_MASK_REASON_EOI_PENDING);
>   }
>   
> @@ -809,13 +811,15 @@ static void pirq_query_unmask(int irq)
>   
>   static void eoi_pirq(struct irq_data *data)
>   {
> -       evtchn_port_t evtchn = evtchn_from_irq(data->irq);
> +       struct irq_info *info = info_for_irq(data->irq);
> +       evtchn_port_t evtchn = info ? info->evtchn : 0;
>         struct physdev_eoi eoi = { .irq = pirq_from_irq(data->irq) };
>         int rc = 0;
>   
>   	if (!VALID_EVTCHN(evtchn))
>                 return;
>   
> +	smp_store_release(&info->is_active, 0);
Would you mind to explain why you are using the release semantics?

It is also not clear to me if there are any expected ordering between 
clearing is_active and clearing the pending bit.
>         clear_evtchn(evtchn);

The 2 lines here seems to be a common pattern in this patch. So I would 
suggest to create a new helper.
>   
>   	if (pirq_needs_eoi(data->irq)) {
> @@ -1640,6 +1644,8 @@ void handle_irq_for_port(evtchn_port_t port, struct 
> evtchn_loop_ctrl *ctrl)
>         }
>   
>   	info = info_for_irq(irq);
> +       if (xchg_acquire(&info->is_active, 1))
> +               return;
>   
>   	if (ctrl->defer_eoi) {
>                 info->eoi_cpu = smp_processor_id();
> @@ -1823,11 +1829,13 @@ static void disable_dynirq(struct irq_data *data)
>   
>   static void ack_dynirq(struct irq_data *data)
>   {
> -       evtchn_port_t evtchn = evtchn_from_irq(data->irq);
> +       struct irq_info *info = info_for_irq(data->irq);
> +       evtchn_port_t evtchn = info ? info->evtchn : 0;
>   
>   	if (!VALID_EVTCHN(evtchn))
>                 return;
>   
> +	smp_store_release(&info->is_active, 0);
>         clear_evtchn(evtchn);
>   }
>   
> @@ -1969,10 +1977,13 @@ static void restore_cpu_ipis(unsigned int cpu)
>   /* Clear an irq's pending state, in preparation for polling on it */
>   void xen_clear_irq_pending(int irq)
>   {
> -       evtchn_port_t evtchn = evtchn_from_irq(irq);
> +       struct irq_info *info = info_for_irq(irq);
> +       evtchn_port_t evtchn = info ? info->evtchn : 0;
>   
> -	if (VALID_EVTCHN(evtchn))
> +       if (VALID_EVTCHN(evtchn)) {
> +               smp_store_release(&info->is_active, 0);
>                 clear_evtchn(evtchn);
> +       }
>   }
>   EXPORT_SYMBOL(xen_clear_irq_pending);
>   void xen_set_irq_pending(int irq)
--
Julien Grall