[PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held

[Jan Beulich <jbeulich@suse.com>]

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.

However, vm_event_disable() frees the structures used by respective
callbacks and isn't otherwise synchronized with invocations of these
callbacks, so maintain a count of in-progress calls, for evtchn_close()
to wait to drop to zero before freeing the port (and dropping the lock).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
Should we make this accounting optional, to be requested through a new
parameter to alloc_unbound_xen_event_channel(), or derived from other
than the default callback being requested?
---
v3: Drain callbacks before proceeding with closing. Re-base.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -397,6 +397,7 @@ static long evtchn_bind_interdomain(evtc
     
     rchn->u.interdomain.remote_dom  = ld;
     rchn->u.interdomain.remote_port = lport;
+    atomic_set(&rchn->u.interdomain.active_calls, 0);
     rchn->state                     = ECS_INTERDOMAIN;
 
     /*
@@ -720,6 +721,10 @@ int evtchn_close(struct domain *d1, int
 
         double_evtchn_lock(chn1, chn2);
 
+        if ( consumer_is_xen(chn1) )
+            while ( atomic_read(&chn1->u.interdomain.active_calls) )
+                cpu_relax();
+
         evtchn_free(d1, chn1);
 
         chn2->state = ECS_UNBOUND;
@@ -781,9 +786,15 @@ int evtchn_send(struct domain *ld, unsig
         rport = lchn->u.interdomain.remote_port;
         rchn  = evtchn_from_port(rd, rport);
         if ( consumer_is_xen(rchn) )
+        {
+            /* Don't keep holding the lock for the call below. */
+            atomic_inc(&rchn->u.interdomain.active_calls);
+            evtchn_read_unlock(lchn);
             xen_notification_fn(rchn)(rd->vcpu[rchn->notify_vcpu_id], rport);
-        else
-            evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn);
+            atomic_dec(&rchn->u.interdomain.active_calls);
+            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);
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -104,6 +104,7 @@ struct evtchn
         } unbound;     /* state == ECS_UNBOUND */
         struct {
             evtchn_port_t  remote_port;
+            atomic_t       active_calls;
             struct domain *remote_dom;
         } interdomain; /* state == ECS_INTERDOMAIN */
         struct {