1 files changed, 7 insertions, 2 deletions

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 2f91e565192..36a5141108d 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -326,8 +326,13 @@ static void xen_spin_unlock(struct raw_spinlock *lock)
         smp_wmb();              /* make sure no writes get moved after unlock */
         xl->lock = 0;           /* release lock */
 
-        /* make sure unlock happens before kick */
-        barrier();
+        /*
+         * Make sure unlock happens before checking for waiting
+         * spinners.  We need a strong barrier to enforce the
+         * write-read ordering to different memory locations, as the
+         * CPU makes no implied guarantees about their ordering.
+         */
+        mb();
 
         if (unlikely(xl->spinners))
                 xen_spin_unlock_slow(xl);