lkml.org 
[lkml]   [2014]   [Jun]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [patch 13/13] tomoyo: Use sensible time interface
From
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 not saying its a problem in this case, but I'm just throwing up
some additional caution since the change you're making isn't
completely equivalent.

thanks
-john


\
 
 \ /
  Last update: 2014-06-12 03:21    [W:0.160 / U:21.432 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site