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

Joshua Hunt
2016-03-12T23:50:36Z

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(). Not only that, but when I
wrote to these proc files in a loop I could easily trigger a soft lockup.

[ 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.
[ 959.788319] NMI watchdog: BUG: soft lockup - CPU#4 stuck for 30s! [swapper/4:0]
[ 959.788325] NMI watchdog: BUG: soft lockup - CPU#5 stuck for 30s! [swapper/5:0]

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

Signed-off-by: Josh Hunt <johunt@akamai.com>
---
kernel/watchdog.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

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