Re: For review: timer_settime.2

[Michael Kerrisk <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>]

Hi Peter,

On 2/15/09, Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org> wrote:
> On Thu, 2009-02-12 at 08:17 +1300, Michael Kerrisk wrote:
>> Hi Peter,
>>
>> Peter Zijlstra wrote:
>> > On Tue, 2009-02-10 at 14:54 +1300, Michael Kerrisk wrote:
>> >>        If the value of the CLOCK_REALTIME clock is adjusted  while  an
>> >>        absolute  timer  based on that clock is armed, then the expira-
>> >>        tion of the timer will be appropriately adjusted.   Adjustments
>> >>        to  the  CLOCK_REALTIME clock have no effect on relative timers
>> >>        based on that clock.
>> >
>> > I cannot find this to be true.
>> >
>> >>From what I can make of the code, clock_settime() ends up calling
>> > do_sys_settimeofday() for CLOCK_REALTIME (and the other clocks).
>> >
>> > It is, however, not treating relative/abs timers any differently.
>> >
>> > Both get converted to an absolute expiration time when set.
>> >
>> > If POSIX mandates that we keep relative timers unchanged when we change
>> > the underlying clock, we'd have to iterate all pending timers and reset
>> > them.
>>
>> The rules do indeed come from POSIX.
>>
>> And indeed the rules do seem to be followed on Linux.  Since I found
>> parts of the code to be hard to track, I also checked things with an
>> example program.  (Ahhh! the pitfalls of reading code to find the
>> truth!)
>
> Ah, quite so. The magic is in hrtimers. Relative timers are ran on clock
> monotonic.
>
> OK I think that was the last bit.
>
> Feel free to add:
>
> Reviewed-by: Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>
>
> To all 4 pathces.

Done.  Thanks for looking the pages over.

Cheers,

Michael

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Michael Kerrisk
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