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, 2 insertions, 4 deletions

diff --git a/drivers/target/loopback/tcm_loop.c b/drivers/target/loopback/tcm_loop.c
index 340de9d92b15..a7f6dc646045 100644
--- a/drivers/target/loopback/tcm_loop.c
+++ b/drivers/target/loopback/tcm_loop.c
@@ -960,8 +960,7 @@ static int tcm_loop_port_link(
 				struct tcm_loop_tpg, tl_se_tpg);
 	struct tcm_loop_hba *tl_hba = tl_tpg->tl_hba;
 
-	atomic_inc(&tl_tpg->tl_tpg_port_count);
-	smp_mb__after_atomic();
+	atomic_inc_mb(&tl_tpg->tl_tpg_port_count);
 	/*
 	 * Add Linux/SCSI struct scsi_device by HCTL
 	 */
@@ -995,8 +994,7 @@ static void tcm_loop_port_unlink(
 	scsi_remove_device(sd);
 	scsi_device_put(sd);
 
-	atomic_dec(&tl_tpg->tl_tpg_port_count);
-	smp_mb__after_atomic();
+	atomic_dec_mb(&tl_tpg->tl_tpg_port_count);
 
 	pr_debug("TCM_Loop_ConfigFS: Port Unlink Successful\n");
 }