[PATCH v2] tty: serial: zs: convert atomic_* to refcount_* APIs for irq_guard

[Deepak R Varma <drv@xxxxxxxxx>]

The refcount_* APIs are designed to address known issues with the
atomic_t APIs for reference counting. They provide following distinct
advantages:
   - protect the reference counters from overflow/underflow
   - avoid use-after-free errors
   - provide improved memory ordering guarantee schemes
   - neater and safer.
Hence, replace the atomic_* APIs by their equivalent refcount_t
API functions.

This patch proposal address the following warnings generated by
the atomic_as_refcounter.cocci coccinelle script
atomic_add_return(-1, ...)

Signed-off-by: Deepak R Varma <drv@xxxxxxxxx>
---

Changes in v2:
   1. Separate the combined change into one variable per patch as
      suggested by gregkh@xxxxxxxxxxxxxxxxxxx
   2. There was additional feedback on validating the change as it appeared to
      modify the existing logic. However, I found that the logic does not
      change with the proposed refcount_* APIs used in this change. Hence that
      feedback is not applied in this version.

Please Note:
   The patch is compile tested using dec_station.defconfig for MIPS architecture.

 drivers/tty/serial/zs.c | 14 +++++---------
 drivers/tty/serial/zs.h |  2 +-
 2 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/serial/zs.c b/drivers/tty/serial/zs.c
index 730c648e32ff..6be9933eff5c 100644
--- a/drivers/tty/serial/zs.c
+++ b/drivers/tty/serial/zs.c
@@ -753,17 +753,15 @@ static int zs_startup(struct uart_port *uport)
 	struct zs_port *zport = to_zport(uport);
 	struct zs_scc *scc = zport->scc;
 	unsigned long flags;
-	int irq_guard;
 	int ret;

-	irq_guard = atomic_add_return(1, &scc->irq_guard);
-	if (irq_guard == 1) {
+	refcount_inc(&scc->irq_guard);
+	if (refcount_read(&scc->irq_guard) == 1) {
 		ret = request_irq(zport->port.irq, zs_interrupt,
 				  IRQF_SHARED, "scc", scc);
 		if (ret) {
-			atomic_add(-1, &scc->irq_guard);
-			printk(KERN_ERR "zs: can't get irq %d\n",
-			       zport->port.irq);
+			refcount_dec(&scc->irq_guard);
+			printk(KERN_ERR "zs: can't get irq %d\n", zport->port.irq);
 			return ret;
 		}
 	}
@@ -806,7 +804,6 @@ static void zs_shutdown(struct uart_port *uport)
 	struct zs_port *zport = to_zport(uport);
 	struct zs_scc *scc = zport->scc;
 	unsigned long flags;
-	int irq_guard;

 	spin_lock_irqsave(&scc->zlock, flags);

@@ -816,8 +813,7 @@ static void zs_shutdown(struct uart_port *uport)

 	spin_unlock_irqrestore(&scc->zlock, flags);

-	irq_guard = atomic_add_return(-1, &scc->irq_guard);
-	if (!irq_guard)
+	if (refcount_dec_and_test(&scc->irq_guard))
 		free_irq(zport->port.irq, scc);
 }

diff --git a/drivers/tty/serial/zs.h b/drivers/tty/serial/zs.h
index 26ef8eafa1c1..bd97b73d7e16 100644
--- a/drivers/tty/serial/zs.h
+++ b/drivers/tty/serial/zs.h
@@ -40,7 +40,7 @@ struct zs_port {
 struct zs_scc {
 	struct zs_port	zport[2];
 	spinlock_t	zlock;
-	atomic_t	irq_guard;
+	refcount_t	irq_guard;
 	int		initialised;
 };

--
2.34.1