Message-ID: <51C2D62A.3060906@ahsoftware.de>
Date:	Thu, 20 Jun 2013 12:15:06 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	John Stultz <john.stultz@...aro.org>
CC:	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [rtc-linux] Re: [PATCH 4/9 RESEND] RFC: timekeeping: introduce
 flag systime_was_set

Am 17.06.2013 20:10, schrieb John Stultz:
> On 06/14/2013 11:01 PM, Alexander Holler wrote:
>> Am 14.06.2013 20:28, schrieb John Stultz:
>>> On 06/14/2013 11:05 AM, Alexander Holler wrote:
>>>> Am 14.06.2013 19:41, schrieb John Stultz:
>>>>> On 06/14/2013 09:52 AM, Alexander Holler wrote:
>>>>>> In order to let an RTC set the time at boot without the problem
>>>>>> that a
>>>>>> second RTC overwrites it, the flag systime_was_set is introduced.
>>>>>>
>>>>>> systime_was_set will be true, if a persistent clock sets the time at
>>>>>> boot,
>>>>>> or if do_settimeofday() is called (e.g. by the RTC subsystem or
>>>>>> userspace).
>>>>>>
>>>>>> Signed-off-by: Alexander Holler <holler@...oftware.de>
>>>>>> ---
>>>>>>   include/linux/time.h      |  6 ++++++
>>>>>>   kernel/time/timekeeping.c | 10 +++++++++-
>>>>>>   2 files changed, 15 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/include/linux/time.h b/include/linux/time.h
>>>>>> index d5d229b..888280f 100644
>>>>>> --- a/include/linux/time.h
>>>>>> +++ b/include/linux/time.h
>>>>>> @@ -129,6 +129,12 @@ extern int update_persistent_clock(struct
>>>>>> timespec now);
>>>>>>   void timekeeping_init(void);
>>>>>>   extern int timekeeping_suspended;
>>>>>> +/*
>>>>>> + * Will be true if the system time was set at least once by
>>>>>> + * a persistent clock, RTC or userspace.
>>>>>> + */
>>>>>> +extern bool systime_was_set;
>>>>>> +
>>>>>
>>>>> Probably should make this static to timekeeping.c and create an
>>>>> accessor
>>>>> function so you don't have to export locking rules on this.
>>>>>
>>>>>
>>>>>>   unsigned long get_seconds(void);
>>>>>>   struct timespec current_kernel_time(void);
>>>>>>   struct timespec __current_kernel_time(void); /* does not take
>>>>>> xtime_lock */
>>>>>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>>>>>> index baeeb5c..07d8531 100644
>>>>>> --- a/kernel/time/timekeeping.c
>>>>>> +++ b/kernel/time/timekeeping.c
>>>>>> @@ -37,6 +37,9 @@ int __read_mostly timekeeping_suspended;
>>>>>>   /* Flag for if there is a persistent clock on this platform */
>>>>>>   bool __read_mostly persistent_clock_exist = false;
>>>>>> +/* Flag for if the system time was set at least once */
>>>>>> +bool __read_mostly systime_was_set;
>>>>>> +
>>>>> Probably should also move this to be part of the timekeeper structure
>>>>> (since it will be protected by the timekeeper lock.
>>>>>
>>>>
>>>> I wanted to avoid locks for this silly flag at all. It is only set
>>>> once at boot (and resume) and set to 0 at suspend. And I don't see any
>>>> possible race condition which could make a lock necessary. Therefor
>>>> I've decided to not use a lock or atomic_* in order to skip any delay
>>>> in setting the time.
>>>
>>> Even so, having random flag variables with special rules being exported
>>> out is likely to cause eventual trouble (someone will mis-use or
>>> overload some meaning on it).
>>>
>>> So at least providing a accessor function for non-timekeeping.c uses
>>> would be good.
>>
>>
>> It's rather hard to misuse a bool (even if a bool in C is just a define).
>
> I'm trying to avoid allowing non-timekeeping users of the value to be
> able to set it.
> By putting the value behind a timekeeping_systime_was_set() accessor,
> and making the boolean value static, we can make sure its properly
> managed by the timekeeping code alone.
>
>> What do you think I should write?
>>
>> void set_systime_was_set(void) and void clear_systime_was_set(void)?
>>
>> And both functions would have to be exported in order to be usable
>> from modules?
>>
>> Or do you think I should write something like that:
>>
>> extern bool foo;
>> inline void set_foo(void) { foo = true};
>> inline void clear_foo(void) { foo = false };
>>
>> That's just silly, sorry to call it such.
>
> No no. I'm only asking that the boolean be static to timekeeping.c and
> an accessor function be used to read it. Since the timekeeping core
> should be managing this value, there should be no reason for any other
> users to be setting or clearing the value.

First you can't make the value static (semantically) because it has to 
be set and cleared from different parts of the kernel. And adding 
accessor functions doesn't help in any way, everyone will still be able 
to set or clear the value (this still is C and  no C++ with classes or 
other encapsulation features). The only thing what will happen with such 
an accessor function is that a small overhead through the then necessary 
function call is introduced.


>
>
>>>> Of course, I might be wrong and there might be a use case where
>>>> multiple things do set the system time concurrently and nothing else
>>>> did set system time before, but I found that extremly unlikely.
>>>
>>> Yea, the condition check and the action won't be both be done under a
>>> lock, so its likely going to be racy anyway.
>>
>> And if there ever will be a race for the first timesource to set this
>> flag (the first time), and something does care about the outtake, the
>> system would be completly broken.
>>
>> In order to keep it simple, I just tread userspace like a RTC of type
>> X and will call them all timesources of type x where a the type is
>> defined by the driver.
>>
>> Let us go through the possible cases:
>>
>> - 2 or more timesources of different type:
>>
>> If the order is undefined and they have to race for which clock might
>> be used for hctosys (and thus for adjusting the time after resume
>> too), the only reason one would want such is for HA purposes. And in
>> case of HA, both clocks must have the same time, so nobody does care
>> about which one will win the race  (=> no race, no lock or atomic_*
>> needed).
>>
>> If the purpose isn't for HA and someone does care about which
>> timesource should be used, the way to do this is to use hctosys=type
>> (or hctosys=none in case of userspace) to define which timesource
>> should be used for hctosys (=> no race, no lock or atomic_* needed).
>>
>> - 2 or more timesources of the same type:
>> There is no possibility to define which one should win the race. Such
>> a system configuration is only usable for HA purposes, so if such
>> exists, nobody cares about the outtake of the race (=> no race, no
>> lock or atomic_* needed).
>>
>
> The race I'm thinking of is you have a system that normally sets the
> time via ntpdate at bootup. Thus they expect the system to always be
> started w/ NTP time (even if the system time was initially set via
> hctosys).
>
> Then because of of some delay in the driver (or because the RTC device
> was plugged in during boot), the hctosys functionality runs just as
> ntpdate is being called.  hctosys sees time has not yet been set and
> reads the RTC hardware time. At this point, ntpdate sets the time to NTP
> time.  Then hctosys completes, setting the time to the RTC time.  This
> results in the system clock being wrong from the user's perspective (as
> they expect it to be set to NTP time).

Therefor there now will be hctosys as a kernel command line parameter. 
Instead of a kernel config option which can't be changed by 99% of all 
Linux users, that option allows ordinary (non kernel compiling) users to 
disable hctosys at all. Something which wasn't possible before without 
recompiling the kernel. And, like before, most RTC drivers will be 
loaded before userspace calls ntp/ntpdate. If not, the system is already 
broken.

And just in case, I've made that possible window for the above race very 
small by checking the flag systime_was_set twice, once before starting 
to read the time and a second time right before the time is set. Ok, the 
race is still there, but as said before, if that problem does exist at 
all, the system would be setup wrong at all.

>
> This is basically what this code is trying to avoid in the first place.
> And I'll grant that its a small race window, but it may lead to
> irregular behavior.
>
> So either we need to document that this race is theoretically possible,
> and explain *why* its safe to ignore it.  Or if we really want to do

I would think that documenting hctosys=none should be enough.

All systems I've seen do load modules very early (before they would 
start anything ntp related). And the new hctosys is done inside the 
registration of the RTC. So if a system is configured in such a way that 
the race really might happen, the system would be broken because the RTC 
would not be there when starting ntp. To conclude, I think the problem 
is highly academic/artificial and, if possible at all, only possible on 
already misconfigured systems during a very, very small window. Therfor 
I still believe that no locking mechanism is needed.

Anyway, I don't care, if you want, I will make an accessor-function with 
locks and an return code for "set" to indicate if it was already set.

We could also request and wait for a third opinion on that topic. As it 
most likely will not end up in any kernel before the end of this year 
(if at all), there is enough time to do so.

Regards,

Alexander Holler

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