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SubjectRe: [patch] fix common mistake in polling loops
Hi!

> task taken from http://kerneljanitors.org/TODO:
>
> A _lot_ of drivers end up caring about absolute time,
> because a _lot_ of
> drivers have a very simple issue like:
>
> - poll this port every 10ms until it returns "ready", or
> until we time
> out after 500ms.
>
> And the thing is, you can do it the stupid way:
>
> for (i = 0; i < 50; i++) {
> if (ready())
> return 0;
> msleep(10);
> }
> .. timeout ..
>
> or you can do it the _right_ way. The stupid way is
> simpler, but anybody
> who doesn't see what the problem is has some serious
> problems in kernel
> programming. Hint: it might not be polling for half a

Well, whoever wrote thi has some serious problems (in attitude
department). *Any* loop you design may take half a minute under
streange circumstances.

> second, it might be
> polling for half a _minute_ for all you know.
>
> In other words, the _right_ way to do this is literally
>
> unsigned long timeout = jiffies +
> msecs_to_jiffies(500);
> for (;;) {
> if (ready())
> return 0;
> if (time_after(timeout, jiffies))
> break;
> msleep(10);
> }
>
> which is unquestionably more complex, yes, but it's more
> complex because
> it is CORRECT!

Original code is correct, too.

Anyway you probably want to hide complexity in some macro if we are
going this way.
Pavel
--
Thanks for all the (sleeping) penguins.
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