linux-kernel - Re: [patch 13/13] tomoyo: Use sensible time interface

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Date:	Thu, 12 Jun 2014 02:36:24 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Kentaro Takeda <takedakn@...data.co.jp>,
	linux-security-module@...r.kernel.org
Subject: Re: [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@...utronix.de> wrote:
> > On Wed, 11 Jun 2014, John Stultz wrote:
> >
> >> On Wed, Jun 11, 2014 at 4:59 PM, Thomas Gleixner <tglx@...utronix.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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