lkml.org 
[lkml]   [2006]   [Aug]   [7]   [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.
    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 01:37    [W:0.024 / U:1.432 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site