This lock is only intended to prevent racing with closing an event
channel - evtchn_set_pending() doesn't require external serialization.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -147,7 +147,7 @@ struct vcpu *alloc_vcpu(
     v->domain = d;
     v->vcpu_id = vcpu_id;
 
-    spin_lock_init(&v->virq_lock);
+    rwlock_init(&v->virq_lock);
 
     tasklet_init(&v->continue_hypercall_tasklet, NULL, 0);
 
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -417,7 +417,7 @@ static long __evtchn_close(struct domain
             if ( v->virq_to_evtchn[chn1->u.virq] != port1 )
                 continue;
             v->virq_to_evtchn[chn1->u.virq] = 0;
-            spin_barrier_irq(&v->virq_lock);
+            write_barrier_irq(&v->virq_lock);
         }
         break;
 
@@ -613,12 +613,11 @@ int guest_enabled_event(struct vcpu *v, 
 
 void send_guest_vcpu_virq(struct vcpu *v, int virq)
 {
-    unsigned long flags;
     int port;
 
     ASSERT(!virq_is_global(virq));
 
-    spin_lock_irqsave(&v->virq_lock, flags);
+    read_lock(&v->virq_lock);
 
     port = v->virq_to_evtchn[virq];
     if ( unlikely(port == 0) )
@@ -627,12 +626,11 @@ void send_guest_vcpu_virq(struct vcpu *v
     evtchn_set_pending(v, port);
 
  out:
-    spin_unlock_irqrestore(&v->virq_lock, flags);
+    read_unlock(&v->virq_lock);
 }
 
 void send_guest_global_virq(struct domain *d, int virq)
 {
-    unsigned long flags;
     int port;
     struct vcpu *v;
     struct evtchn *chn;
@@ -646,7 +644,7 @@ void send_guest_global_virq(struct domai
     if ( unlikely(v == NULL) )
         return;
 
-    spin_lock_irqsave(&v->virq_lock, flags);
+    read_lock(&v->virq_lock);
 
     port = v->virq_to_evtchn[virq];
     if ( unlikely(port == 0) )
@@ -656,7 +654,7 @@ void send_guest_global_virq(struct domai
     evtchn_set_pending(d->vcpu[chn->notify_vcpu_id], port);
 
  out:
-    spin_unlock_irqrestore(&v->virq_lock, flags);
+    read_unlock(&v->virq_lock);
 }
 
 int send_guest_pirq(struct domain *d, int pirq)
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -344,6 +344,20 @@ int _rw_is_write_locked(rwlock_t *lock)
     return _raw_rw_is_write_locked(&lock->raw);
 }
 
+void _write_barrier(rwlock_t *lock)
+{
+    check_lock(&lock->debug);
+    do { mb(); } while ( _raw_rw_is_locked(&lock->raw) );
+}
+
+void _write_barrier_irq(rwlock_t *lock)
+{
+    unsigned long flags;
+    local_irq_save(flags);
+    _write_barrier(lock);
+    local_irq_restore(flags);
+}
+
 #ifdef LOCK_PROFILE
 
 struct lock_profile_anc {
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -157,9 +157,12 @@ struct vcpu 
     unsigned long    pause_flags;
     atomic_t         pause_count;
 
-    /* IRQ-safe virq_lock protects against delivering VIRQ to stale evtchn. */
+    /*
+     * Writer-side-IRQ-safe virq_lock protects against delivering VIRQ
+     * to stale evtchn.
+     */
     u16              virq_to_evtchn[NR_VIRQS];
-    spinlock_t       virq_lock;
+    rwlock_t         virq_lock;
 
     /* Bitmask of CPUs on which this VCPU may run. */
     cpumask_t        cpu_affinity;
--- a/xen/include/xen/spinlock.h
+++ b/xen/include/xen/spinlock.h
@@ -170,6 +170,9 @@ void _write_unlock_irqrestore(rwlock_t *
 int _rw_is_locked(rwlock_t *lock);
 int _rw_is_write_locked(rwlock_t *lock);
 
+void _write_barrier(rwlock_t *lock);
+void _write_barrier_irq(rwlock_t *lock);
+
 #define spin_lock(l)                  _spin_lock(l)
 #define spin_lock_irq(l)              _spin_lock_irq(l)
 #define spin_lock_irqsave(l, f)       ((f) = _spin_lock_irqsave(l))
@@ -223,4 +226,7 @@ int _rw_is_write_locked(rwlock_t *lock);
 #define rw_is_locked(l)               _rw_is_locked(l)
 #define rw_is_write_locked(l)         _rw_is_write_locked(l)
 
+#define write_barrier(l)              _write_barrier(l)
+#define write_barrier_irq(l)          _write_barrier_irq(l)
+
 #endif /* __SPINLOCK_H__ */