atomics: eliminate mb_read/mb_set

qatomic_mb_read and qatomic_mb_set were the very first atomic primitives
introduced for QEMU; their semantics are unclear and they provide a false
sense of safety.

The last use of qatomic_mb_read() has been removed, so delete it.
qatomic_mb_set() instead can survive as an optimized
qatomic_set()+smp_mb(), similar to Linux's smp_store_mb(), but
rename it to qatomic_set_mb() to match the order of the two
operations.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 5 insertions, 12 deletions

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index f85834ee8b..d95612f7a0 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -259,24 +259,17 @@
 # define smp_mb__after_rmw() smp_mb()
 #endif
 
-/* qatomic_mb_read/set semantics map Java volatile variables. They are
- * less expensive on some platforms (notably POWER) than fully
- * sequentially consistent operations.
- *
- * As long as they are used as paired operations they are safe to
- * use. See docs/devel/atomics.rst for more discussion.
+/*
+ * On some architectures, qatomic_set_mb is more efficient than a store
+ * plus a fence.
  */
 
-#define qatomic_mb_read(ptr)                             \
-    qatomic_load_acquire(ptr)
-
 #if !defined(QEMU_SANITIZE_THREAD) && \
     (defined(__i386__) || defined(__x86_64__) || defined(__s390x__))
-/* This is more efficient than a store plus a fence.  */
-# define qatomic_mb_set(ptr, i) \
+# define qatomic_set_mb(ptr, i) \
     ({ (void)qatomic_xchg(ptr, i); smp_mb__after_rmw(); })
 #else
-# define qatomic_mb_set(ptr, i) \
+# define qatomic_set_mb(ptr, i) \
    ({ qatomic_store_release(ptr, i); smp_mb(); })
 #endif