linux-kernel - [PATCH] watchdog: don't run proc_watchdog

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Message-Id: <1457826627-21727-1-git-send-email-johunt@akamai.com>
Date:	Sat, 12 Mar 2016 18:50:26 -0500
From:	Joshua Hunt <johunt@...mai.com>
To:	akpm@...ux-foundation.org, uobergfe@...hat.com, dzickus@...hat.com
Cc:	linux-kernel@...r.kernel.org, Joshua Hunt <johunt@...mai.com>
Subject: [PATCH] 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(). 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@...mai.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