membarrier: introduce qemu/sys_membarrier.h

This new header file provides heavy-weight "global" memory barriers that
enforce memory ordering on each running thread belonging to the current
process.  For now, use a dummy implementation that issues memory barriers
on both sides (matching what QEMU has been doing so far).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 625f09ac09..22876d1428 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -27,6 +27,7 @@
 #include "qemu/thread.h"
 #include "qemu/queue.h"
 #include "qemu/atomic.h"
+#include "qemu/sys_membarrier.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -82,7 +83,7 @@ static inline void rcu_read_lock(void)
     atomic_set(&p_rcu_reader->ctr, ctr);
 
     /* Write p_rcu_reader->ctr before reading RCU-protected pointers.  */
-    smp_mb();
+    smp_mb_placeholder();
 }
 
 static inline void rcu_read_unlock(void)
@@ -96,13 +97,13 @@ static inline void rcu_read_unlock(void)
 
     /* Ensure that the critical section is seen to precede the
      * store to p_rcu_reader->ctr.  Together with the following
-     * smp_mb(), this ensures writes to p_rcu_reader->ctr
+     * smp_mb_placeholder(), this ensures writes to p_rcu_reader->ctr
      * are sequentially consistent.
      */
     atomic_store_release(&p_rcu_reader->ctr, 0);
 
     /* Write p_rcu_reader->ctr before reading p_rcu_reader->waiting.  */
-    smp_mb();
+    smp_mb_placeholder();
     if (unlikely(atomic_read(&p_rcu_reader->waiting))) {
         atomic_set(&p_rcu_reader->waiting, false);
         qemu_event_set(&rcu_gp_event);