lkml.org 
[lkml]   [2006]   [Aug]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [KJ] [patch] fix common mistake in polling loops
Hi!

> >> >> Well, whoever wrote thi has some serious problems (in attitude
> >> >> department). *Any* loop you design may take half a minute under
> >> >> streange circumstances.
> >>
> >> 6.
> >> common mistake in polling loops [from Linus]:
> >
> >Yes, Linus was wrong here. Or more precisely, he's right original code
> >is broken, but his suggested "fix" is worse than the original.
> >
> > unsigned long timeout = jiffies + HZ/2;
> > for (;;) {
> > if (ready())
> > return 0;
> >[IMAGINE HALF A SECOND DELAY HERE]
> > if (time_after(timeout, jiffies))
> > break;
> > msleep(10);
> > }
> >
> >Oops.
> >
> >> >Actually it may be broken, depending on use. In some cases this loop
> >> >may want to poll the hardware 50 times, 10msec appart... and your loop
> >> >can poll it only once in extreme conditions.
> >> >
> >> >Actually your loop is totally broken, and may poll only once (without
> >> >any delay) and then directly timeout :-P -- that will break _any_
> >> >user.
> >>
> >> The Idea is that we are checking some event in external hardware that
> >> we know will complete in a given time (This time is not dependant on
> >> system activity but is fixed). After that time if the event has not
> >> happened we know something has borked.
> >
> >But you have to make sure YOU CHECK READY AFTER THE TIMEOUT. Linus'
> >code does not do that.
>
> Sorry I did not realise that was your problem with the code.
> Would it help if we just explicitly added a
>
> if (ready())
> return 0;
>
> after the loop, in the example code? so people wont miss adding
> something like that in?

Yes, that would do the trick.
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-08-08 11:21    [W:0.057 / U:0.196 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site