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SubjectRe: [patch 13/13] tomoyo: Use sensible time interface
On Wed, 11 Jun 2014, John Stultz wrote:
> On Wed, Jun 11, 2014 at 5:22 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Wed, 11 Jun 2014, John Stultz wrote:
> >
> >> On Wed, Jun 11, 2014 at 4:59 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >> > There is no point in calling gettimeofday if only the seconds part of
> >> > the timespec is used. Use get_seconds() instead. It's not only the
> >> > proper interface it's also faster.
> >>
> >> My only caution here is you only get tick-granular time here. So if
> >> the second rolled over after the last tick, you'd get the previous
> >> second when you call get_seconds(). This can cause some surprising
> >> effects if the get_seconds() return value is mixed with clocksource
> >> granular gettimeofday() calls.
> >
> > If the whole thing only cares about the seconds value, then where is
> > the problem?
> >
> > Even if you call gettimeofday() then you still can observe this
> >
> > gettimeofday(ts)
> > ts.tv_sec = 99
> > ts.tv_nsec = 999999999
> >
> > So if you readout the related value ONE nanosecond later, then this
> > value will have
> > ts.tv_sec = 100
> > ts.tv_nsec = 0
> >
> > So what's the point? The tomoyo code chose to take seconds granular
> > time stamps for whatever reasons. So it should be able to deal with
> > that, right?
>
> No, the problem I'm warning about is if they were using gettimeofday()
> elsewhere in relation to those timestamps, they could see something
> like:
>
> do_gettimeofday() { 99, 888....}
> get_seconds() { 99 }
> do_gettimeofday() { 99, 999....}
> get_seconds() { 99 }
> do_gettimeofday() { 100, 000....}
> get_seconds() { 99 }
> do_gettimeofday() { 100, 011....}
> get_seconds() { 100 }
>
> This is the same problem people come across occasionally if they call
> gettimeofday, then create a file and fret that the file's timestamp
> seems to be before the gettimefoday call, and its all due to comparing
> timestamps with different granularities.
>
I'm aware of that, but there are only two places in that code which
deal with time and both are calling do_gettimeofday and both just use
the tv_sec part of it. And both of them are statistics.

One part of says clearly:

* I don't use atomic operations because race condition is not fatal.

Thanks,

tglx




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