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

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Message-ID: <AANLkTikK9m7USWqN9QZnjnNbHen9bVyFt6xiGxNG4OyY@mail.gmail.com>
Date:	Wed, 9 Jun 2010 22:55:13 -0700
From:	Salman Qazi <sqazi@...gle.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, Jun 9, 2010 at 10:17 PM, Michel Lespinasse <walken@...gle.com> wrote:
> On Wed, Jun 9, 2010 at 5:20 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> 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.
>
> As shown by the three compares version, the question here does not depend on
> the max_pid value. So, we can replace max_pid with any integer >= max_pid
> without changing the result. This is nice because, replacing max_pid with
> 2^32, the expression simplifies to:
>
> (unsigned)(a - base) < (unsigned)(b - base)
>
> I think I like this form after all :)

I don't mind doing this, if nobody disagrees.  Should be fine with a
comment on top explaining the intention?

>
> --
> Michel "Walken" Lespinasse
> A program is never fully debugged until the last user dies.
>
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