linux-kernel - Re: [tip:timers/urgent] tick: Fix oneshot broadcast setup really

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Date:	Wed, 18 Apr 2012 21:21:02 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, suresh.b.siddha@...el.com,
	svenjoac@....de, rjw@...k.pl, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/urgent] tick: Fix oneshot broadcast setup really

On Wednesday 18 April 2012 09:01 PM, Thomas Gleixner wrote:
> On Wed, 18 Apr 2012, Santosh Shilimkar wrote:
>> On Wednesday 18 April 2012 06:49 PM, Shilimkar, Santosh wrote:
>>> On Wed, Apr 18, 2012 at 5:37 PM, tip-bot for Thomas Gleixner
>>> <tglx@...utronix.de> wrote:
>>>> Commit-ID:  b435092f70ec5ebbfb6d075d5bf3c631b49a51de
>>>> Gitweb:     http://git.kernel.org/tip/b435092f70ec5ebbfb6d075d5bf3c631b49a51de
>>>> Author:     Thomas Gleixner <tglx@...utronix.de>
>>>> AuthorDate: Wed, 18 Apr 2012 12:08:23 +0200
>>>> Committer:  Thomas Gleixner <tglx@...utronix.de>
>>>> CommitDate: Wed, 18 Apr 2012 14:00:56 +0200
>>>>
>>>> tick: Fix oneshot broadcast setup really
>>>>
>>>> Sven Joachim reported, that suspend/resume on rc3 trips over a NULL
>>>> pointer dereference. Linus spotted the clockevent handler being NULL.
>>>>
>>>> commit fa4da365b(clockevents: tTack broadcast device mode change in
>>>> tick_broadcast_switch_to_oneshot()) tried to fix a problem with the
>>>> broadcast device setup, which was introduced in commit 77b0d60c5(
>>>> clockevents: Leave the broadcast device in shutdown mode when not
>>>> needed).
>>>>
>>>> The initial commit avoided to set up the broadcast device when no
>>>> broadcast request bits were set, but that left the broadcast device
>>>> disfunctional. In consequence deep idle states which need the
>>>> broadcast device were not woken up.
>>>>
>>>> commit fa4da365b tried to fix that by initializing the state of the
>>>> broadcast facility, but that missed the fact, that nothing initializes
>>>> the event handler and some other state of the underlying clock event
>>>> device.
>>>>
>>>> The fix is to revert both commits and make only the mode setting of
>>>> the clock event device conditional on the state of active broadcast
>>>> users.
>>>>
>>>> That initializes everything except the low level device mode, but this
>>>> happens when the broadcast functionality is invoked by deep idle.
>>>>
>>>> Reported-and-tested-by: Sven Joachim <svenjoac@....de>
>>>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>>>> Cc: Rafael J. Wysocki <rjw@...k.pl>
>>>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>>>> Cc: Suresh Siddha <suresh.b.siddha@...el.com>
>>>> Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1204181205540.2542@ionos
>>>>
>>>> ---
>>>>  kernel/time/tick-broadcast.c |    7 +------
>>>>  1 files changed, 1 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
>>>> index bf57abd..119aca5 100644
>>>> --- a/kernel/time/tick-broadcast.c
>>>> +++ b/kernel/time/tick-broadcast.c
>>>> @@ -531,7 +531,6 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
>>>>                int was_periodic = bc->mode == CLOCK_EVT_MODE_PERIODIC;
>>>>
>>>>                bc->event_handler = tick_handle_oneshot_broadcast;
>>>> -               clockevents_set_mode(bc, CLOCK_EVT_MODE_ONESHOT);
>>>>
>>>>                /* Take the do_timer update */
>>>>                tick_do_timer_cpu = cpu;
>>>> @@ -549,6 +548,7 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
>>>>                           to_cpumask(tmpmask));
>>>>
>>>>                if (was_periodic && !cpumask_empty(to_cpumask(tmpmask))) {
>>>> +                       clockevents_set_mode(bc, CLOCK_EVT_MODE_ONESHOT);
>>
>> For some reason above if() check fails in my case, so broadcast device
>> never set to ONESHOT mode. That explains the problem I am seeing on
>> OMAP with the $subject patch. At this point of time bc->mode is
>> CLOCK_EVT_MODE_UNUSED.
> 
> Darn, crap. I wonder how that works on x86
> 
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index bf57abd..e8f5479 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -373,6 +373,9 @@ static int tick_broadcast_set_event(ktime_t expires, int force)
>  {
>  	struct clock_event_device *bc = tick_broadcast_device.evtdev;
>  
> +	if (bc->mode != CLOCK_EVT_MODE_ONESHOT)
> +		clockevents_set_mode(bc, CLOCK_EVT_MODE_ONESHOT);
> +
>  	return clockevents_program_event(bc, expires, force);
>  }
>  
Appending above change to $subject patch makes things work
nicely again.

Regards
Santosh
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