[PATCH v2] watchdog: don't run proc_watchdog_update if new value is same as old

[Joshua Hunt]

While working on a script to restore all sysctl params before a series of
tests I found that writing any value into the
/proc/sys/kernel/{nmi_watchdog,soft_watchdog,watchdog,watchdog_thresh}
causes them to call proc_watchdog_update().

[  955.756196] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
[  955.765994] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
[  955.774619] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
[  955.783182] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.

There doesn't appear to be a reason for doing this work every time a write
occurs, so only do it when the values change.

Signed-off-by: Josh Hunt <johunt@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 4.1.x-
---
 kernel/watchdog.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

v2: Update changelog to no longer include mention of soft lockup, and add stable.

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index b3ace6e..9acb29f 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -923,6 +923,9 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 		 * both lockup detectors are disabled if proc_watchdog_update()
 		 * returns an error.
 		 */
+		if (old == new)
+			goto out;
+
 		err = proc_watchdog_update();
 	}
 out:
@@ -967,7 +970,7 @@ int proc_soft_watchdog(struct ctl_table *table, int write,
 int proc_watchdog_thresh(struct ctl_table *table, int write,
 			 void __user *buffer, size_t *lenp, loff_t *ppos)
 {
-	int err, old;
+	int err, old, new;
 
 	get_online_cpus();
 	mutex_lock(&watchdog_proc_mutex);
@@ -987,6 +990,10 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 	/*
 	 * Update the sample period. Restore on failure.
 	 */
+	new = ACCESS_ONCE(watchdog_thresh);
+	if (old == new)
+		goto out;
+
 	set_sample_period();
 	err = proc_watchdog_update();
 	if (err) {
-- 
1.7.9.5