Messages in this thread | | | Date | Tue, 8 Aug 2006 01:34:53 +0200 | From | Pavel Machek <> | Subject | Re: [KJ] [patch] fix common mistake in polling loops |
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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 -- (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/
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