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SubjectRe: [PATCH v2 3/8] xen/events: avoid handling the same event on two cpus at the same time
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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@xen.org>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> 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

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