xen/events: avoid handling the same event on two cpus at the same time

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.

Cc: stable@vger.kernel.org
Reported-by: Julien Grall <julien@xen.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Link: https://lore.kernel.org/r/20210306161833.4552-4-jgross@suse.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/events/events_base.c

@@ -103,6 +103,7 @@ 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 */
@@ -810,6 +811,12 @@ static void xen_evtchn_close(evtchn_port_t port)
 		BUG();
 }
 
+static void event_handler_exit(struct irq_info *info)
+{
+	smp_store_release(&info->is_active, 0);
+	clear_evtchn(info->evtchn);
+}
+
 static void pirq_query_unmask(int irq)
 {
 	struct physdev_irq_status_query irq_status;
@@ -828,14 +835,15 @@ 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;
 
-	clear_evtchn(evtchn);
+	event_handler_exit(info);
 
 	if (pirq_needs_eoi(data->irq)) {
 		rc = HYPERVISOR_physdev_op(PHYSDEVOP_eoi, &eoi);
@@ -1666,6 +1674,8 @@ 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;
 
 	dev = (info->type == IRQT_EVTCHN) ? info->u.interdomain : NULL;
 	if (dev)
@@ -1853,12 +1863,11 @@ 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);
-
-	if (!VALID_EVTCHN(evtchn))
-		return;
+	struct irq_info *info = info_for_irq(data->irq);
+	evtchn_port_t evtchn = info ? info->evtchn : 0;
 
-	clear_evtchn(evtchn);
+	if (VALID_EVTCHN(evtchn))
+		event_handler_exit(info);
 }
 
 static void mask_ack_dynirq(struct irq_data *data)
@@ -1874,7 +1883,7 @@ static void lateeoi_ack_dynirq(struct irq_data *data)
 
 	if (VALID_EVTCHN(evtchn)) {
 		do_mask(info, EVT_MASK_REASON_EOI_PENDING);
-		clear_evtchn(evtchn);
+		event_handler_exit(info);
 	}
 }
 
@@ -1885,7 +1894,7 @@ static void lateeoi_mask_ack_dynirq(struct irq_data *data)
 
 	if (VALID_EVTCHN(evtchn)) {
 		do_mask(info, EVT_MASK_REASON_EXPLICIT);
-		clear_evtchn(evtchn);
+		event_handler_exit(info);
 	}
 }
 
@@ -1998,10 +2007,11 @@ 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))
-		clear_evtchn(evtchn);
+		event_handler_exit(info);
 }
 EXPORT_SYMBOL(xen_clear_irq_pending);
 void xen_set_irq_pending(int irq)