[PATCH v4 04/10] 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>
---
v4: Go back to v2.
v3: Drain callbacks before proceeding with closing. Re-base.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -767,9 +767,18 @@ 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);
+            evtchn_read_unlock(lchn);
+            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);