lib/ovs-atomic-gcc4+: Use 'volatile' to enforce memory access.

Use 'volatile' to enforce a new memory access on each lockless atomic store and read. Without this a loop consisting of an atomic_read with memory_order_relaxed would be simply optimized away. Also, using volatile is cheaper than adding a full compiler barrier (also) in that case. This use of a volatile cast mirrors the Linux kernel ACCESS_ONCE macro. Without this change the more rigorous atomic test cases introduced in a following patch will hang due to the atomic accesses being optimized away. Signed-off-by: Jarno Rajahalme <[email protected]> Acked-by: Ben Pfaff <[email protected]>
hugb · Aug 5, 2014 · 860f83f · 860f83f
1 parent 9164558
commit 860f83f
Showing 1 changed file with 2 additions and 2 deletions.
diff --git a/lib/ovs-atomic-gcc4+.h b/lib/ovs-atomic-gcc4+.h
@@ -83,7 +83,7 @@ atomic_signal_fence(memory_order order)
                                                         \
         if (IS_LOCKLESS_ATOMIC(*dst__)) {               \
             atomic_thread_fence(ORDER);                 \
-            *dst__ = src__;                             \
+            *(typeof(*DST) volatile *)dst__ = src__;    \
             atomic_thread_fence_if_seq_cst(ORDER);      \
         } else {                                        \
             atomic_store_locked(dst__, src__);          \
@@ -99,7 +99,7 @@ atomic_signal_fence(memory_order order)
                                                         \
         if (IS_LOCKLESS_ATOMIC(*src__)) {               \
             atomic_thread_fence_if_seq_cst(ORDER);      \
-            *dst__ = *src__;                            \
+            *dst__ = *(typeof(*SRC) volatile *)src__;   \
         } else {                                        \
             atomic_read_locked(src__, dst__);           \
         }                                               \