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

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Date:   Sun, 14 Feb 2021 21:34:31 +0000
From:   Julien Grall <julien@....org>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 3/8] xen/events: avoid handling the same event on two
 cpus at the same time

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@....org>
> Signed-off-by: Juergen Gross <jgross@...e.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