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

From: Salman Qazi
Date: Wed Jun 09 2010 - 20:09:03 EST

On Wed, Jun 9, 2010 at 3:27 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 9 Jun 2010, Linus Torvalds wrote:
>> Quite possibly. I'd worry about the overflow case a bit, but it's
>> certainly going to get the right value when base << MAX_INT.
> Having given it a couple of seconds more thought, I don't think there is
> an overflow case either. All of a/b/base are guaranteed to be non-negative
> (or our pid code is in worse trouble anyway), so there is no overflow
> possible. So yes. Just comparing a-base < b-base should always be safe

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. 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?
>                        Linus
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