linux-kernel - Re: [patch] fix common mistake in polling loops

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sat, 5 Aug 2006 11:40:52 +0000
From:	Pavel Machek <pavel@....cz>
To:	Zed 0xff <zed.0xff@...il.com>
Cc:	kernel-janitors@...l.org, linux-kernel@...r.kernel.org
Subject: Re: [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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/