Patch "hrtimer: Reset hrtimer cpu base proper on CPU hotplug" has been added to the 4.4-stable tree

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

hrtimer: Reset hrtimer cpu base proper on CPU hotplug

to the 4.4-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:
hrtimer-reset-hrtimer-cpu-base-proper-on-cpu-hotplug.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Thomas Gleixner <[email protected]>
Date: Fri, 26 Jan 2018 14:54:32 +0100
Subject: hrtimer: Reset hrtimer cpu base proper on CPU hotplug

From: Thomas Gleixner <[email protected]>

commit d5421ea43d30701e03cadc56a38854c36a8b4433 upstream.

The hrtimer interrupt code contains a hang detection and mitigation mechanism, which prevents that a long delayed hrtimer interrupt causes a continous retriggering of interrupts which prevent the system from making progress. If a hang is detected then the timer hardware is programmed with a certain delay into the future and a flag is set in the hrtimer cpu base which prevents newly enqueued timers from reprogramming the timer hardware prior to the chosen delay. The subsequent hrtimer interrupt after the delay clears the flag and resumes normal operation.

If such a hang happens in the last hrtimer interrupt before a CPU is unplugged then the hang_detected flag is set and stays that way when the CPU is plugged in again. At that point the timer hardware is not armed and it cannot be armed because the hang_detected flag is still active, so nothing clears that flag. As a consequence the CPU does not receive hrtimer interrupts and no timers expire on that CPU which results in RCU stalls and other malfunctions.

Clear the flag along with some other less critical members of the hrtimer cpu base to ensure starting from a clean state when a CPU is plugged in.

Thanks to Paul, Sebastian and Anna-Maria for their help to get down to the root cause of that hard to reproduce heisenbug. Once understood it's trivial and certainly justifies a brown paperbag.

---
 kernel/time/hrtimer.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -669,7 +669,9 @@ static void hrtimer_reprogram(struct hrt
 static inline void hrtimer_init_hres(struct hrtimer_cpu_base *base)
 {
 	base->expires_next.tv64 = KTIME_MAX;
+	base->hang_detected = 0;
 	base->hres_active = 0;
+	base->next_timer = NULL;
 }
 
 /*

@@ -1615,6 +1617,7 @@ static void init_hrtimers_cpu(int cpu)
 		timerqueue_init_head(&cpu_base->clock_base[i].active);
 	}
 
+	cpu_base->active_bases = 0;
 	cpu_base->cpu = cpu;
 	hrtimer_init_hres(cpu_base);
 }

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

queue-4.4/prevent-timer-value-0-for-mwaitx.patch queue-4.4/x86-ioapic-fix-incorrect-pointers-in-ioapic_setup_resources.patch queue-4.4/x86-asm-32-make-sync_core-handle-missing-cpuid-on-all-32-bit-kernels.patch queue-4.4/timers-plug-locking-race-vs.-timer-migration.patch queue-4.4/x86-cpu-intel-introduce-macros-for-intel-family-numbers.patch queue-4.4/x86-retpoline-fill-rsb-on-context-switch-for-affected-cpus.patch queue-4.4/revert-module-add-retpoline-tag-to-vermagic.patch queue-4.4/x86-microcode-intel-extend-bdw-late-loading-further-with-llc-size-check.patch queue-4.4/time-avoid-undefined-behaviour-in-ktime_add_safe.patch queue-4.4/sched-deadline-use-the-revised-wakeup-rule-for-suspending-constrained-dl-tasks.patch queue-4.4/hrtimer-reset-hrtimer-cpu-base-proper-on-cpu-hotplug.patch