[PATCH v3 2/5] evtchn: avoid access tearing for ->virq_to_evtchn[] accesses

[Jan Beulich <jbeulich@xxxxxxxx>]

Use {read,write}_atomic() to exclude any eventualities, in particular
observing that accesses aren't all happening under a consistent lock.

Requested-by: Julien Grall <julien@xxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: New.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -446,7 +446,7 @@ int evtchn_bind_virq(evtchn_bind_virq_t
 
     spin_lock(&d->event_lock);
 
-    if ( v->virq_to_evtchn[virq] != 0 )
+    if ( read_atomic(&v->virq_to_evtchn[virq]) )
         ERROR_EXIT(-EEXIST);
 
     if ( port != 0 )
@@ -474,7 +474,8 @@ int evtchn_bind_virq(evtchn_bind_virq_t
 
     evtchn_write_unlock(chn);
 
-    v->virq_to_evtchn[virq] = bind->port = port;
+    bind->port = port;
+    write_atomic(&v->virq_to_evtchn[virq], port);
 
  out:
     spin_unlock(&d->event_lock);
@@ -660,9 +661,9 @@ int evtchn_close(struct domain *d1, int
     case ECS_VIRQ:
         for_each_vcpu ( d1, v )
         {
-            if ( v->virq_to_evtchn[chn1->u.virq] != port1 )
+            if ( read_atomic(&v->virq_to_evtchn[chn1->u.virq]) != port1 )
                 continue;
-            v->virq_to_evtchn[chn1->u.virq] = 0;
+            write_atomic(&v->virq_to_evtchn[chn1->u.virq], 0);
             spin_barrier(&v->virq_lock);
         }
         break;
@@ -801,7 +802,7 @@ bool evtchn_virq_enabled(const struct vc
     if ( virq_is_global(virq) && v->vcpu_id )
         v = domain_vcpu(v->domain, 0);
 
-    return v->virq_to_evtchn[virq];
+    return read_atomic(&v->virq_to_evtchn[virq]);
 }
 
 void send_guest_vcpu_virq(struct vcpu *v, uint32_t virq)
@@ -814,7 +815,7 @@ void send_guest_vcpu_virq(struct vcpu *v
 
     spin_lock_irqsave(&v->virq_lock, flags);
 
-    port = v->virq_to_evtchn[virq];
+    port = read_atomic(&v->virq_to_evtchn[virq]);
     if ( unlikely(port == 0) )
         goto out;
 
@@ -843,7 +844,7 @@ void send_guest_global_virq(struct domai
 
     spin_lock_irqsave(&v->virq_lock, flags);
 
-    port = v->virq_to_evtchn[virq];
+    port = read_atomic(&v->virq_to_evtchn[virq]);
     if ( unlikely(port == 0) )
         goto out;