linux-kernel - Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

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Message-ID: <alpine.LFD.2.00.1006091719100.4506@i5.linux-foundation.org>
Date:	Wed, 9 Jun 2010 17:20:21 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Salman Qazi <sqazi@...gle.com>
cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	mingo@...e.hu, linux-kernel@...r.kernel.org, tytso@...gle.com
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be 
 reused immediately.



On Wed, 9 Jun 2010, Salman Qazi wrote:
> 
> I don't think this gives the right answer in the a < base < b  case.
> Here a - base < 0 and
> b - base > 0.  But we really want b to be before a, since a has rolled
> over further than b.

Right you are.

> I think the right solution is comparing (a - base + max_pid) % max_pid 
> with (b - base + max_pid) % max_pid.  Am I correct or deluded? .

That would work, but it would be horrible. Just use the three compares 
version: doing a integer 'mod' operator is _way_ more expensive.

			Linus
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