On Mon, Jan 29, 2018 at 03:20:32PM +0100, Sebastian Andrzej Siewior wrote:
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 Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801261447590.2067@nanos [bigeasy: backport to v3.18, drop ->next_timer it was introduced later]
Thanks for the backport, now queued up.
greg k-h