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

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Date:	Wed, 18 Apr 2012 19:48:33 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, suresh.b.siddha@...el.com,
	svenjoac@....de, tglx@...utronix.de, rjw@...k.pl
CC:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/urgent] tick: Fix oneshot broadcast setup really

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.

Regards
Santosh
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