Messages in this thread | | | Date | Mon, 7 Aug 2006 19:53:55 -0700 | From | "Om N." <> | Subject | Re: [KJ] [patch] fix common mistake in polling loops |
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On 8/7/06, Darren Jenkins <darrenrjenkins@gmail.com> wrote: > G'day > > On 8/8/06, Pavel Machek <pavel@suse.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? - 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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