Patch "hrtimer: Reset hrtimer cpu base proper on CPU hotplug" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

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 4.9-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:
     hrtimer-reset-hrtimer-cpu-base-proper-on-cpu-hotplug.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 <stable@vger.kernel.org> know about it.


>From d5421ea43d30701e03cadc56a38854c36a8b4433 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Fri, 26 Jan 2018 14:54:32 +0100
Subject: hrtimer: Reset hrtimer cpu base proper on CPU hotplug

From: Thomas Gleixner <tglx@linutronix.de>

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.

Fixes: 41d2e4949377 ("hrtimer: Tune hrtimer_interrupt hang logic")
Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Sewior <bigeasy@linutronix.de>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801261447590.2067@nanos
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -652,7 +652,9 @@ 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;
 }
 
 /*
@@ -1610,6 +1612,7 @@ int hrtimers_prepare_cpu(unsigned int cp
 		timerqueue_init_head(&cpu_base->clock_base[i].active);
 	}
 
+	cpu_base->active_bases = 0;
 	cpu_base->cpu = cpu;
 	hrtimer_init_hres(cpu_base);
 	return 0;


Patches currently in stable-queue which might be from tglx@linutronix.de are

queue-4.9/prevent-timer-value-0-for-mwaitx.patch
queue-4.9/perf-x86-amd-power-do-not-load-amd-power-module-on-amd-platforms.patch
queue-4.9/x86-asm-32-make-sync_core-handle-missing-cpuid-on-all-32-bit-kernels.patch
queue-4.9/revert-module-add-retpoline-tag-to-vermagic.patch
queue-4.9/x86-microcode-intel-extend-bdw-late-loading-further-with-llc-size-check.patch
queue-4.9/hrtimer-reset-hrtimer-cpu-base-proper-on-cpu-hotplug.patch