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Date: Mon, 7 Aug 2006 19:53:55 -0700
From: "Om N." <xhandle@...il.com>
To: unlisted-recipients:; (no To-header on input)
Cc: kernel-janitors@...l.org, linux-kernel@...r.kernel.org
Subject: Re: [KJ] [patch] fix common mistake in polling loops
On 8/7/06, Darren Jenkins <darrenrjenkins@...il.com> wrote:
> G'day
>
> On 8/8/06, Pavel Machek <pavel@...e.cz> wrote:
> > 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.
> > Pavel
>
>
> Sorry I did not realise that was your problem with the code.
> Would it help if we just explicitly added a
>
unsigned long timeout = jiffies + HZ/2;
for (;;) {
if (ready())
return 0;
[IMAGINE HALF A SECOND DELAY HERE]
if (time_after(timeout, jiffies)) {
if (ready())
return 0;
break;
}
msleep(10);
}
Wouldn't this be better than adding a check after the break of loop?
> if (ready())
> return 0;
>
> after the loop, in the example code? so people wont miss adding
> something like that in?
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