Re: [PATCH] tick: Fix hang caused by hrtimer in broadcast mode

[Thomas Gleixner]

On Fri, 24 Apr 2015, Andreas Sandberg wrote:

> The hrtimer callback in the hrtimer's tick broadcast code sometimes
> incorrectly ends up scheduling events at the current tick causing the
> kernel to hang servicing the same hrtimer forever. This typically
> happens when a device is swapped out by
> tick_install_broadcast_device(), which replaces the event handler with
> clock_events_handle_noop() and sets the device mode to
> CLOCK_EVT_MODE_UNUSED. If the timer is scheduled when this happens,
> the next_event field will not be updated and the hrtimer ends up being
> restarted at the current tick. To prevent this from happening, only
> try to restart the hrtimer if the broadcast clock event device is in
> one of the active modes and try to cancel the timer when entering the
> CLOCK_EVT_MODE_UNUSED mode.
> 
> Signed-off-by: Andreas Sandberg <andreas.sandberg@xxxxxxx>
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> Tested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> ---
>  kernel/time/tick-broadcast-hrtimer.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
> index 6aac4be..a20c605 100644
> --- a/kernel/time/tick-broadcast-hrtimer.c
> +++ b/kernel/time/tick-broadcast-hrtimer.c
> @@ -22,6 +22,7 @@ static void bc_set_mode(enum clock_event_mode mode,
>  			struct clock_event_device *bc)
>  {
>  	switch (mode) {
> +	case CLOCK_EVT_MODE_UNUSED:
>  	case CLOCK_EVT_MODE_SHUTDOWN:
>  		/*
>  		 * Note, we cannot cancel the timer here as we might
> @@ -99,10 +100,14 @@ static enum hrtimer_restart bc_handler(struct hrtimer *t)
>  {
>  	ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
>  
> -	if (ce_broadcast_hrtimer.next_event.tv64 == KTIME_MAX)
> +	switch (ce_broadcast_hrtimer.mode) {
> +	case CLOCK_EVT_MODE_PERIODIC:

That case is pointless. The mode cannot be periodic simply because
.features lacks the periodic flag.

Thanks,

	tglx
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