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From: John Stultz <john.stultz@linaro.org>
To: Richard Cochran <richardcochran@gmail.com>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Prarit Bhargava <prarit@redhat.com>,
	Sharvil Nanavati <sharvil@google.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH] alarmtimer: Fix bug where relative alarm timers were treated as absolute
Date: Tue, 8 Jul 2014 09:34:32 +0200	[thread overview]
Message-ID: <CALAqxLUixKYE1z8GcZzkfrSNqZwtJNBqyYuxo78t+UtswmC68Q@mail.gmail.com> (raw)
In-Reply-To: <20140708064204.GD3977@localhost.localdomain>

On Tue, Jul 8, 2014 at 8:42 AM, Richard Cochran
<richardcochran@gmail.com> wrote:
> On Mon, Jul 07, 2014 at 02:06:11PM -0700, John Stultz wrote:
>> @@ -597,8 +602,16 @@ static int alarm_timer_set(struct k_itimer *timr, int flags,
>>
>>       /* start the timer */
>>       timr->it.alarm.interval = timespec_to_ktime(new_setting->it_interval);
>> -     alarm_start(&timr->it.alarm.alarmtimer,
>> -                     timespec_to_ktime(new_setting->it_value));
>> +     exp = timespec_to_ktime(new_setting->it_value);
>> +     /* Convert (if necessary) to absolute time */
>> +     if (flags != TIMER_ABSTIME) {
>> +             ktime_t now;
>> +
>> +             now = alarm_bases[timr->it.alarm.alarmtimer.type].gettime();
>> +             exp = ktime_add(now, exp);
>> +     }
>> +
>> +     alarm_start(&timr->it.alarm.alarmtimer, exp);
>
> Nothing protects 'exp' from becoming invalid before queuing the alarm,
> if the time base is reset on another cpu. Or would that be harmless
> here?

Hrmn.. So that's a separate question, but a good one to validate as well.

common_timer_set() has a similar behavior where the return value of
hrtimer_start_expires() isn't passed up. This is because the userspace
behavior is that if the time has already passed, the timer should fire
immediately.

If you look in __hrtimer_start_range_ns(), you'll see the chunk of
logic at the bottom which (if I'm reading it right) raises the softirq
(to fire the timer) if our timer is the earliest and
enqueue_reprogram fails (due to the clockevent logic returning ETIME
due to the time being in the past).

So its definitely subtle but looks like its ok. But I'll add a
validation test to be sure I'm not missing anything there as well.

thanks
-john

  reply	other threads:[~2014-07-08  7:34 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-07 21:06 John Stultz
2014-07-08  6:42 ` Richard Cochran
2014-07-08  7:34   ` John Stultz [this message]
2014-07-08  8:52 ` [tip:timers/urgent] " tip-bot for John Stultz

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