atomics: do not emit consume barrier for atomic_rcu_read

Currently we emit a consume-load in atomic_rcu_read. Because of
limitations in current compilers, this is overkill for non-Alpha hosts
and it is only useful to make Thread Sanitizer work.

This patch leaves the consume-load in atomic_rcu_read when
compiling with Thread Sanitizer enabled, and resorts to a
relaxed load + smp_read_barrier_depends otherwise.

On an RMO host architecture, such as aarch64, the performance
improvement of this change is easily measurable. For instance,
qht-bench performs an atomic_rcu_read on every lookup. Performance
before and after applying this patch:

$ tests/qht-bench -d 5 -n 1
Before: 9.78 MT/s
After: 10.96 MT/s

Backports commit 15487aa132109891482f79d78a30d6cfd465a391 from qemu

qemu/include/qemu/atomic.h

@@ -70,12 +70,22 @@ void _ReadWriteBarrier(void);
     __atomic_store(ptr, &_val, __ATOMIC_RELAXED); \
 } while(0)
 
-/* Atomic RCU operations imply weak memory barriers */
+/* See above: most compilers currently treat consume and acquire the
+ * same, but this slows down atomic_rcu_read unnecessarily.
+ */
+#ifdef __SANITIZE_THREAD__
+#define atomic_rcu_read__nocheck(ptr, valptr)           \
+    __atomic_load(ptr, valptr, __ATOMIC_CONSUME);
+#else
+#define atomic_rcu_read__nocheck(ptr, valptr)           \
+    __atomic_load(ptr, valptr, __ATOMIC_RELAXED);       \
+    smp_read_barrier_depends();
+#endif
 
 #define atomic_rcu_read(ptr)                      \
     ({                                            \
     typeof(*ptr) _val;                            \
-     __atomic_load(ptr, &_val, __ATOMIC_CONSUME); \
+    atomic_rcu_read__nocheck(ptr, &_val);         \
     _val;                                         \
     })