target: encapsulate smp_mb__after_atomic()

The target code has a rather generous helping of smp_mb__after_atomic()
throughout the code base.  Most atomic operations were followed by one
and none were preceded by smp_mb__before_atomic(), nor accompanied by a
comment explaining the need for a barrier.

Instead of trying to prove for every case whether or not it is needed,
this patch introduces atomic_inc_mb() and atomic_dec_mb(), which
explicitly include the memory barriers before and after the atomic
operation.  For now they are defined in a target header, although they
could be of general use.

Most of the existing atomic/mb combinations were replaced by the new
helpers.  In a few cases the atomic was sandwiched in
spin_lock/spin_unlock and I simply removed the barrier.

I suspect that in most cases the correct conversion would have been to
drop the barrier.  I also suspect that a few cases exist where a) the
barrier was necessary and b) a second barrier before the atomic would
have been necessary and got added by this patch.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/target/target_core_base.h b/include/target/target_core_base.h
index 9ec9864ecf38..b106240d8385 100644
--- a/include/target/target_core_base.h
+++ b/include/target/target_core_base.h
@@ -903,4 +903,18 @@ struct se_wwn {
 	struct config_group	fabric_stat_group;
 };
 
+static inline void atomic_inc_mb(atomic_t *v)
+{
+	smp_mb__before_atomic();
+	atomic_inc(v);
+	smp_mb__after_atomic();
+}
+
+static inline void atomic_dec_mb(atomic_t *v)
+{
+	smp_mb__before_atomic();
+	atomic_dec(v);
+	smp_mb__after_atomic();
+}
+
 #endif /* TARGET_CORE_BASE_H */