[Qemu-devel] [PATCH v2 2/3] atomics: emit an smp_read_barrier_depends() barrier only for Sparc and Thread Sanitizer

["Emilio G. Cota" <cota@braap.org>]

For correctness, smp_read_barrier_depends() is only required to
emit a barrier on Sparc hosts. However, we are currently emitting
a consume fence unconditionally.

Fix it by keeping the consume fence if we're compiling with Thread
Sanitizer, since this might help prevent false warnings. Otherwise,
only emit the barrier for Sparc hosts. Note that we still guarantee
that smp_read_barrier_depends() is a compiler barrier.

Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 include/qemu/atomic.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 5bc4d6c..4a4f2fb 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -36,7 +36,14 @@
 #define smp_wmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_RELEASE); barrier(); })
 #define smp_rmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_ACQUIRE); barrier(); })
 
+#if defined(__SANITIZE_THREAD__)
 #define smp_read_barrier_depends() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); barrier(); })
+#elsif defined(__alpha__)
+#define smp_read_barrier_depends()   asm volatile("mb":::"memory")
+#else
+#define smp_read_barrier_depends()   barrier()
+#endif
+
 
 /* Weak atomic operations prevent the compiler moving other
  * loads/stores past the atomic operation load/store. However there is
-- 
2.5.0