The patch below does not apply to the 3.18-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

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: [PATCH] hrtimer: Reset hrtimer cpu base proper on CPU hotplug

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 Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801261447590.2067@nanos

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index d32520840fde..aa9d2a2b1210 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -655,7 +655,9 @@ static void hrtimer_reprogram(struct hrtimer *timer, static inline void hrtimer_init_hres(struct hrtimer_cpu_base *base) { base->expires_next = KTIME_MAX; + base->hang_detected = 0; base->hres_active = 0; + base->next_timer = NULL; }

/* @@ -1589,6 +1591,7 @@ int hrtimers_prepare_cpu(unsigned 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); return 0;