watchdog: don't run proc_watchdog_update if new value is same as old

commit a1ee1932 upstream.

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().

  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
  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@akamai.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Reviewed-by: Aaron Tomlin <atomlin@redhat.com>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

kernel/watchdog.c

@@ -823,6 +823,9 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 		 * Update the run state of the lockup detectors.
 		 * Restore 'watchdog_enabled' on failure.
 		 */
+		if (old == new)
+			goto out;
+
 		err = proc_watchdog_update();
 		if (err)
 			watchdog_enabled = old;
@@ -868,7 +871,7 @@ 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;
 
 	mutex_lock(&watchdog_proc_mutex);
 
@@ -882,6 +885,10 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 	 * Update the sample period.
 	 * Restore 'watchdog_thresh' on failure.
 	 */
+	new = ACCESS_ONCE(watchdog_thresh);
+	if (old == new)
+		goto out;
+
 	set_sample_period();
 	err = proc_watchdog_update();
 	if (err)