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

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>
Cc: <stable@vger.kernel.org>	[4.1.x+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index b3ace6ebbba3..9acb29f280ec 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) {