[PATCH v4 02/10] evtchn: bind-interdomain doesn't need to hold both domains' event locks

[Jan Beulich <jbeulich@xxxxxxxx>]

The local domain's lock is needed for the port allocation, but for the
remote side the per-channel lock is sufficient. The per-channel locks
then need to be acquired slightly earlier, though.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v4: New.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -355,18 +355,7 @@ static long evtchn_bind_interdomain(evtc
     if ( (rd = rcu_lock_domain_by_id(rdom)) == NULL )
         return -ESRCH;
 
-    /* Avoid deadlock by first acquiring lock of domain with smaller id. */
-    if ( ld < rd )
-    {
-        spin_lock(&ld->event_lock);
-        spin_lock(&rd->event_lock);
-    }
-    else
-    {
-        if ( ld != rd )
-            spin_lock(&rd->event_lock);
-        spin_lock(&ld->event_lock);
-    }
+    spin_lock(&ld->event_lock);
 
     if ( (lport = get_free_port(ld)) < 0 )
         ERROR_EXIT(lport);
@@ -375,15 +364,19 @@ static long evtchn_bind_interdomain(evtc
     if ( !port_is_valid(rd, rport) )
         ERROR_EXIT_DOM(-EINVAL, rd);
     rchn = evtchn_from_port(rd, rport);
+
+    double_evtchn_lock(lchn, rchn);
+
     if ( (rchn->state != ECS_UNBOUND) ||
          (rchn->u.unbound.remote_domid != ld->domain_id) )
         ERROR_EXIT_DOM(-EINVAL, rd);
 
     rc = xsm_evtchn_interdomain(XSM_HOOK, ld, lchn, rd, rchn);
     if ( rc )
+    {
+        double_evtchn_unlock(lchn, rchn);
         goto out;
-
-    double_evtchn_lock(lchn, rchn);
+    }
 
     lchn->u.interdomain.remote_dom  = rd;
     lchn->u.interdomain.remote_port = rport;
@@ -407,8 +400,6 @@ static long evtchn_bind_interdomain(evtc
  out:
     check_free_port(ld, lport);
     spin_unlock(&ld->event_lock);
-    if ( ld != rd )
-        spin_unlock(&rd->event_lock);
     
     rcu_unlock_domain(rd);