time_after - what on earth???

time_after - what on earth???

[Adrian McMenamin]

OK, why does this line occasionally return true:

	if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))

while this one never does (no other changes made):

if  ((maple_dev->interval > 0) && (time_after(jiffies, maple_dev->when)))


Is this a gcc issue or what?

Re: time_after - what on earth???

[Rene Herman]

On 09/12/2007 12:05 AM, Adrian McMenamin wrote:

> OK, why does this line occasionally return true:
> 
> 	if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
> 
> while this one never does (no other changes made):
> 
> if  ((maple_dev->interval > 0) && (time_after(jiffies, maple_dev->when)))

Is maple_dev->when an unsigned long?

Rene.

Re: time_after - what on earth???

[Adrian McMenamin]

On 11/09/2007, Rene Herman <rene.herman@gmail.com> wrote:
> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
>
> > OK, why does this line occasionally return true:
> >
> >       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
> >
> > while this one never does (no other changes made):
> >
> > if  ((maple_dev->interval > 0) && (time_after(jiffies, maple_dev->when)))
>
> Is maple_dev->when an unsigned long?
>
Yes. Does that make a difference?

Re: time_after - what on earth???

[Rene Herman]

On 09/12/2007 12:15 AM, Adrian McMenamin wrote:

> On 11/09/2007, Rene Herman <rene.herman@gmail.com> wrote:
>> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
>>
>>> OK, why does this line occasionally return true:
>>>
>>>       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
>>>
>>> while this one never does (no other changes made):
>>>
>>> if  ((maple_dev->interval > 0) && (time_after(jiffies, maple_dev->when)))
>> Is maple_dev->when an unsigned long?
>>
> Yes. Does that make a difference?

If it had been a signed type, it could've wrapped to something you didn't 
expect, explaining the difference at least...

With an unsigned long, the only diference should be that time_after() deals 
with jiffie wrapping which I assume is not an actual problem here. I'll 
retreat into the shades again... ;-(

Rene.

Re: time_after - what on earth???

[Björn Steinbrink]

On 2007.09.12 00:19:09 +0200, Rene Herman wrote:
> On 09/12/2007 12:15 AM, Adrian McMenamin wrote:
>
>> On 11/09/2007, Rene Herman <rene.herman@gmail.com> wrote:
>>> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
>>>
>>>> OK, why does this line occasionally return true:

What exactly is "occassionally"?  Does it happen more than once per
boot? If not, and it happens after a certain time after booting, it
might be wrapping of the jiffie counter (see below).

>>>>
>>>>       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
>>>>
>>>> while this one never does (no other changes made):
>>>>
>>>> if  ((maple_dev->interval > 0) && (time_after(jiffies, 
>>>> maple_dev->when)))
>>> Is maple_dev->when an unsigned long?
>>>
>> Yes. Does that make a difference?
>
> If it had been a signed type, it could've wrapped to something you didn't 
> expect, explaining the difference at least...
>
> With an unsigned long, the only diference should be that time_after() deals 
> with jiffie wrapping which I assume is not an actual problem here. I'll 
> retreat into the shades again... ;-(

If "occasionally" is limited to once per boot, it might be jiffie
wrapping. IIRC jiffies are initialized so that they wrap after about 5
minutes of uptime to reveal such bugs without forcing you to wait for
ages just to have the counter wrap for the first time.

Björn

Re: time_after - what on earth???

[Adrian McMenamin]

On 12/09/2007, Björn Steinbrink <B.Steinbrink@gmx.de> wrote:
> On 2007.09.12 00:19:09 +0200, Rene Herman wrote:
> > On 09/12/2007 12:15 AM, Adrian McMenamin wrote:
> >
> >> On 11/09/2007, Rene Herman <rene.herman@gmail.com> wrote:
> >>> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
> >>>
> >>>> OK, why does this line occasionally return true:
>
> What exactly is "occassionally"?  Does it happen more than once per
> boot? If not, and it happens after a certain time after booting, it
> might be wrapping of the jiffie counter (see below).
>
> >>>>
> >>>>       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
> >>>>
> >>>> while this one never does (no other changes made):
> >>>>
> >>>> if  ((maple_dev->interval > 0) && (time_after(jiffies,
> >>>> maple_dev->when)))
> >>> Is maple_dev->when an unsigned long?
> >>>
> >> Yes. Does that make a difference?
> >
> > If it had been a signed type, it could've wrapped to something you didn't
> > expect, explaining the difference at least...
> >
> > With an unsigned long, the only diference should be that time_after() deals
> > with jiffie wrapping which I assume is not an actual problem here. I'll
> > retreat into the shades again... ;-(
>
> If "occasionally" is limited to once per boot, it might be jiffie
> wrapping. IIRC jiffies are initialized so that they wrap after about 5
> minutes of uptime to reveal such bugs without forcing you to wait for
> ages just to have the counter wrap for the first time.
>

No, I mean "works properly" - ie occasionally evaluates as true

Re: time_after - what on earth???

[Björn Steinbrink]

On 2007.09.12 00:10:19 +0100, Adrian McMenamin wrote:
> On 12/09/2007, Björn Steinbrink <B.Steinbrink@gmx.de> wrote:
> > On 2007.09.12 00:19:09 +0200, Rene Herman wrote:
> > > On 09/12/2007 12:15 AM, Adrian McMenamin wrote:
> > >
> > >> On 11/09/2007, Rene Herman <rene.herman@gmail.com> wrote:
> > >>> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
> > >>>
> > >>>> OK, why does this line occasionally return true:
> >
> > What exactly is "occassionally"?  Does it happen more than once per
> > boot? If not, and it happens after a certain time after booting, it
> > might be wrapping of the jiffie counter (see below).
> >
> > >>>>
> > >>>>       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
> > >>>>
> > >>>> while this one never does (no other changes made):
> > >>>>
> > >>>> if  ((maple_dev->interval > 0) && (time_after(jiffies,
> > >>>> maple_dev->when)))
> > >>> Is maple_dev->when an unsigned long?
> > >>>
> > >> Yes. Does that make a difference?
> > >
> > > If it had been a signed type, it could've wrapped to something you didn't
> > > expect, explaining the difference at least...
> > >
> > > With an unsigned long, the only diference should be that time_after() deals
> > > with jiffie wrapping which I assume is not an actual problem here. I'll
> > > retreat into the shades again... ;-(
> >
> > If "occasionally" is limited to once per boot, it might be jiffie
> > wrapping. IIRC jiffies are initialized so that they wrap after about 5
> > minutes of uptime to reveal such bugs without forcing you to wait for
> > ages just to have the counter wrap for the first time.
> >
> 
> No, I mean "works properly" - ie occasionally evaluates as true

Ehrm, yeah, I somehow parsed that as if it had a negation in there.

Anyway, I looked up the patches you posted. "when" is initialized to 0
and only changed if the above condition evaluates to true. Now,
time_after and "<" have different points at which "future" and "past"
are separated. time_after splits (about) equally between future and
past, so 0 can be either, depending on the value of jiffies. But for "<"
0 is almost always in the past, except for the seldom event of jiffies
being 0.

Now, given that jiffies start out at a huge value to make the counter
wrap around early, 0 happens to be in the "future" for time_after, until
the wrap around occurs. So in your case, you just might have to wait
those 5 minutes to get the working behaviour, instead of the common case
in which it breaks after that time ;-)

A fix would likely initialize "when" to jiffies.

Björn

Re: time_after - what on earth???

[Adrian McMenamin]

On 12/09/2007, Björn Steinbrink <B.Steinbrink@gmx.de> wrote:

>
> A fix would likely initialize "when" to jiffies.
>
> Björn
>

Thanks, I'll try that :)

Re: time_after - what on earth???

[Rene Herman]

On 09/12/2007 01:09 AM, Björn Steinbrink wrote:
> On 2007.09.12 00:19:09 +0200, Rene Herman wrote:
>> On 09/12/2007 12:15 AM, Adrian McMenamin wrote:
>>
>>> On 11/09/2007, Rene Herman <rene.herman@gmail.com> wrote:
>>>> On 09/12/2007 12:05 AM, Adrian McMenamin wrote:
>>>>
>>>>> OK, why does this line occasionally return true:
> 
> What exactly is "occassionally"?  Does it happen more than once per
> boot? If not, and it happens after a certain time after booting, it
> might be wrapping of the jiffie counter (see below).
> 
>>>>>       if ((maple_dev->interval > 0) && (jiffies >maple_dev->when))
>>>>>
>>>>> while this one never does (no other changes made):
>>>>>
>>>>> if  ((maple_dev->interval > 0) && (time_after(jiffies, 
>>>>> maple_dev->when)))
>>>> Is maple_dev->when an unsigned long?
>>>>
>>> Yes. Does that make a difference?
>> If it had been a signed type, it could've wrapped to something you didn't 
>> expect, explaining the difference at least...
>>
>> With an unsigned long, the only diference should be that time_after() deals 
>> with jiffie wrapping which I assume is not an actual problem here. I'll 
>> retreat into the shades again... ;-(
> 
> If "occasionally" is limited to once per boot, it might be jiffie
> wrapping. IIRC jiffies are initialized so that they wrap after about 5
> minutes of uptime to reveal such bugs without forcing you to wait for
> ages just to have the counter wrap for the first time.

Yes, but if jiifie wrapping was the problem, I'd expect the contrary 
behaviour with the time_after() one hitting while the > one does not.

Rene.