[PATCH RFC v2 8/8] evtchn: don't call Xen consumer callback with per-channel lock held

[Jan Beulich <jbeulich@xxxxxxxx>]

While there don't look to be any problems with this right now, the lock
order implications from holding the lock can be very difficult to follow
(and may be easy to violate unknowingly). The present callbacks don't
(and no such callback should) have any need for the lock to be held.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
TODO: vm_event_disable() frees the structures used by respective
      callbacks - need to either use call_rcu() for freeing, or maintain
      a count of in-progress calls, for evtchn_close() to wait to drop
      to zero before dropping the lock / returning.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -763,9 +763,18 @@ int evtchn_send(struct domain *ld, unsig
         rport = lchn->u.interdomain.remote_port;
         rchn  = evtchn_from_port(rd, rport);
         if ( consumer_is_xen(rchn) )
-            xen_notification_fn(rchn)(rd->vcpu[rchn->notify_vcpu_id], rport);
-        else
-            evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn);
+        {
+            /* Don't keep holding the lock for the call below. */
+            xen_event_channel_notification_t fn = xen_notification_fn(rchn);
+            struct vcpu *rv = rd->vcpu[rchn->notify_vcpu_id];
+
+            rcu_lock_domain(rd);
+            spin_unlock_irqrestore(&lchn->lock, flags);
+            fn(rv, rport);
+            rcu_unlock_domain(rd);
+            return 0;
+        }
+        evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn);
         break;
     case ECS_IPI:
         evtchn_port_set_pending(ld, lchn->notify_vcpu_id, lchn);