Patch "kernel/watchdog: prevent false hardlockup on overloaded system" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

kernel/watchdog: prevent false hardlockup on overloaded system

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kernel-watchdog-prevent-false-hardlockup-on-overloaded-system.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Don Zickus <[email protected]>
Date: Tue, 24 Jan 2017 15:17:53 -0800
Subject: kernel/watchdog: prevent false hardlockup on overloaded system

From: Don Zickus <[email protected]>

[ Upstream commit b94f51183b0617e7b9b4fb4137d4cf1cab7547c2 ]

On an overloaded system, it is possible that a change in the watchdog threshold can be delayed long enough to trigger a false positive.

This can easily be achieved by having a cpu spinning indefinitely on a task, while another cpu updates watchdog threshold.

What happens is while trying to park the watchdog threads, the hrtimers on the other cpus trigger and reprogram themselves with the new slower watchdog threshold. Meanwhile, the nmi watchdog is still programmed with the old faster threshold.

Because the one cpu is blocked, it prevents the thread parking on the other cpus from completing, which is needed to shutdown the nmi watchdog and reprogram it correctly. As a result, a false positive from the nmi watchdog is reported.

Fix this by setting a park_in_progress flag to block all lockups until the parking is complete.

Fix provided by Ulrich Obergfell.

---
 include/linux/nmi.h   |    1 +
 kernel/watchdog.c     |    9 +++++++++
 kernel/watchdog_hld.c |    3 +++
 3 files changed, 13 insertions(+)

@@ -110,6 +110,7 @@ extern int watchdog_user_enabled;
 extern int watchdog_thresh;
 extern unsigned long watchdog_enabled;
 extern unsigned long *watchdog_cpumask_bits;
+extern atomic_t watchdog_park_in_progress;
 #ifdef CONFIG_SMP
 extern int sysctl_softlockup_all_cpu_backtrace;
 extern int sysctl_hardlockup_all_cpu_backtrace;

@@ -49,6 +49,8 @@ unsigned long *watchdog_cpumask_bits = c
 #define for_each_watchdog_cpu(cpu) \
 	for_each_cpu_and((cpu), cpu_online_mask, &watchdog_cpumask)
 
+atomic_t watchdog_park_in_progress = ATOMIC_INIT(0);
+
 /*
  * The 'watchdog_running' variable is set to 1 when the watchdog threads
  * are registered/started and is set to 0 when the watchdog threads are

@@ -260,6 +262,9 @@ static enum hrtimer_restart watchdog_tim
 	int duration;
 	int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;
 
+	if (atomic_read(&watchdog_park_in_progress) != 0)
+		return HRTIMER_NORESTART;
+
 	/* kick the hardlockup detector */
 	watchdog_interrupt_count();
 

@@ -467,12 +472,16 @@ static int watchdog_park_threads(void)
 {
 	int cpu, ret = 0;
 
+	atomic_set(&watchdog_park_in_progress, 1);
+
 	for_each_watchdog_cpu(cpu) {
 		ret = kthread_park(per_cpu(softlockup_watchdog, cpu));
 		if (ret)
 			break;
 	}
 
+	atomic_set(&watchdog_park_in_progress, 0);
+
 	return ret;
 }
 

@@ -84,6 +84,9 @@ static void watchdog_overflow_callback(s
 	/* Ensure the watchdog never gets throttled */
 	event->hw.interrupts = 0;
 
+	if (atomic_read(&watchdog_park_in_progress) != 0)
+		return;
+
 	if (__this_cpu_read(watchdog_nmi_touch) == true) {
 		__this_cpu_write(watchdog_nmi_touch, false);
 		return;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kernel-watchdog.c-move-shared-definitions-to-nmi.h.patch queue-4.9/kernel-watchdog-prevent-false-hardlockup-on-overloaded-system.patch queue-4.9/kernel-watchdog.c-move-hardlockup-detector-to-separate-file.patch