Arul,
On Thu, 5 Sep 2019, Arul Jeniston wrote:
> When we adjust the date setting using date command we observed
> 'timerfd_read()' on CLOCK_REALTIME (TFD_TIMER_ABSTIME flag is set)
> returns 0.
> we don't see any hardware influence here and we are able to recreate
> it consistently. Is it expected? if yes, isn't it something to be
> documented in timerfd read() man page?
It's expected, yes. Simply because it hits the following condition:
armtimer(T1)
settime(T1 + X) --> causes timer to fire
wakeup reader
settime(T0)
read number of intervals: 0
i.e. timer did not expire
Changing the return value to 1 would be just a cosmetic workaround. We
could also jump back and wait again. But that's all not consistent because
armtimer(T1)
settime(T1 + X) --> causes timer to fire
wakeup reader
read number of intervals: 1
settime(T0)
user space reads time and figures that
the returned tick is bogus.
So I rather change the documentation (this applies only to CLOCK_REALTIME
and CLOCK_REALTIME_ALARM) and explain the rationale.
For applications which care about notifications when the time was set,
timerfd_settime() provides TFD_TIMER_CANCEL_ON_SET which causes the timer
to be canceled when time is set and returns -ECANCELED from the
read. That's unambiguous.
Thanks,
tglx
