[PATCH v2] tty: serial: dz: 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/dz.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/dz.c b/drivers/tty/serial/dz.c
index b70edc248f8b..0aa59a9beeb7 100644
--- a/drivers/tty/serial/dz.c
+++ b/drivers/tty/serial/dz.c
@@ -46,7 +46,6 @@
 #include <linux/tty.h>
 #include <linux/tty_flip.h>

-#include <linux/atomic.h>
 #include <linux/refcount.h>
 #include <linux/io.h>
 #include <asm/bootinfo.h>
@@ -77,7 +76,7 @@ struct dz_port {
 struct dz_mux {
 	struct dz_port		dport[DZ_NB_PORT];
 	refcount_t		map_guard;
-	atomic_t		irq_guard;
+	refcount_t		irq_guard;
 	int			initialised;
 };

@@ -400,18 +399,16 @@ static int dz_startup(struct uart_port *uport)
 	struct dz_port *dport = to_dport(uport);
 	struct dz_mux *mux = dport->mux;
 	unsigned long flags;
-	int irq_guard;
 	int ret;
 	u16 tmp;

-	irq_guard = atomic_add_return(1, &mux->irq_guard);
-	if (irq_guard != 1)
+	refcount_inc(&mux->irq_guard);
+	if (refcount_read(&mux->irq_guard) != 1)
 		return 0;

-	ret = request_irq(dport->port.irq, dz_interrupt,
-			  IRQF_SHARED, "dz", mux);
+	ret = request_irq(dport->port.irq, dz_interrupt, IRQF_SHARED, "dz", mux);
 	if (ret) {
-		atomic_add(-1, &mux->irq_guard);
+		refcount_dec(&mux->irq_guard);
 		printk(KERN_ERR "dz: Cannot get IRQ %d!\n", dport->port.irq);
 		return ret;
 	}
@@ -441,15 +438,13 @@ static void dz_shutdown(struct uart_port *uport)
 	struct dz_port *dport = to_dport(uport);
 	struct dz_mux *mux = dport->mux;
 	unsigned long flags;
-	int irq_guard;
 	u16 tmp;

 	spin_lock_irqsave(&dport->port.lock, flags);
 	dz_stop_tx(&dport->port);
 	spin_unlock_irqrestore(&dport->port.lock, flags);

-	irq_guard = atomic_add_return(-1, &mux->irq_guard);
-	if (!irq_guard) {
+	if (refcount_dec_and_test(&mux->irq_guard)) {
 		/* Disable interrupts.  */
 		tmp = dz_in(dport, DZ_CSR);
 		tmp &= ~(DZ_RIE | DZ_TIE);
--
2.34.1