Re: [KJ] [patch] fix common mistake in polling loops

From: Pavel Machek <pavel@ucw.cz>
To: Darren Jenkins <darrenrjenkins@gmail.com>
Cc: torvalds@osdl.org, Zed 0xff <zed.0xff@gmail.com>,
	kernel-janitors@osdl.org, linux-kernel@vger.kernel.org
Subject: Re: [KJ] [patch] fix common mistake in polling loops
Date: Tue, 08 Aug 2006 09:19:11 +0000	[thread overview]
Message-ID: <20060808091911.GA4245@elf.ucw.cz> (raw)
In-Reply-To: <82faac5b0608071753q71050d72uadcf55bc1e54f30e@mail.gmail.com>

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
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