Re: [PATCH v3] time: Fix incorrect sleeptime injection when suspend fails

[Mukesh Ojha <mojha@codeaurora.org>]

Hi John,

Thanks for your response
Please find my comments inline.

On 7/11/2018 1:43 AM, John Stultz wrote:
> On Fri, Jul 6, 2018 at 6:17 AM, Mukesh Ojha <mojha@codeaurora.org> wrote:
>> Currently, there exists a corner case assuming when there is
>> only one clocksource e.g RTC, and system failed to go to
>> suspend mode. While resume rtc_resume() injects the sleeptime
>> as timekeeping_rtc_skipresume() returned 'false' (default value
>> of sleeptime_injected) due to which we can see mismatch in
>> timestamps.
>>
>> This issue can also come in a system where more than one
>> clocksource are present and very first suspend fails.
>>
>> Fix this by handling `sleeptime_injected` flag properly.
>>
>> Success case:
>> ------------
>>                                          {sleeptime_injected=false}
>> rtc_suspend() => timekeeping_suspend() => timekeeping_resume() =>
>>
>> (sleeptime injected)
>>   rtc_resume()
>>
>> Failure case:
>> ------------
>>           {failure in sleep path} {sleeptime_injected=false}
>> rtc_suspend()     =>          rtc_resume()
>>
>> sleeptime injected again which was not required as the suspend failed)
>>
>> Originally-by: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>
>> ---
>> Changes in v3:
>>   * Updated commit subject and description.
>>   * Updated the patch as per the fix given by Thomas Gleixner.
>>
>> Changes in v2:
>>   * Updated the commit text.
>>   * Removed extra variable and used the earlier static
>>     variable 'sleeptime_injected'.
>>
>>   kernel/time/timekeeping.c | 21 ++++++++++++++++++---
>>   1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>> index 4786df9..32ae9ae 100644
>> --- a/kernel/time/timekeeping.c
>> +++ b/kernel/time/timekeeping.c
>> @@ -1510,8 +1510,20 @@ void __weak read_boot_clock64(struct timespec64 *ts)
>>          ts->tv_nsec = 0;
>>   }
>>
>> -/* Flag for if timekeeping_resume() has injected sleeptime */
>> -static bool sleeptime_injected;
>> +/*
>> + * Flag reflecting whether timekeeping_resume() has injected sleeptime.
>> + *
>> + * The flag starts of true and is only cleared when a suspend reaches
>> + * timekeeping_suspend(), timekeeping_resume() sets it when the timekeeper
>> + * clocksource is not stopping across suspend and has been used to update
>> + * sleep time. If the timekeeper clocksource has stopped then the flag
>> + * stays false and is used by the RTC resume code to decide whether sleep
>> + * time must be injected and if so the flag gets set then.
>> + *
>> + * If a suspend fails before reaching timekeeping_resume() then the flag
>> + * stays true and prevents erroneous sleeptime injection.
>> + */
>> +static bool sleeptime_injected = true;
> I worry this upside-down logic is too subtle to be easily reasoned
> about, and will just lead to future mistakes.
>
> Can we instead call this "suspend_timing_needed" and only set it to
> true when we don't inject any sleep time on resume?

I did not get your point "only set it to true when we don't inject any 
sleep time on resume? "
How do we know  this ?
This question itself depends on the "sleeptime_injected" if it is true 
means no need to inject else need to inject.

Also, we need to make this variable back and forth true, false; suspends 
path ensures it to make it false.

Just to add here there are already two path where `sleeptime_injected` 
set to true one from
NON-stop clocksource and other from persistant clock and the RTC one was 
missing, so we are adding
with this patch.

Cheers,
-Mukesh