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

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Message-ID: <20100610203815.GA15868@thunk.org>
Date:	Thu, 10 Jun 2010 16:38:15 -0400
From:	tytso@....edu
To:	Salman <sqazi@...gle.com>
Cc:	peterz@...radead.org, linux-kernel@...r.kernel.org,
	tytso@...gle.com, akpm@...ux-foundation.org, walken@...gle.com,
	torvalds@...ux-foundation.org, mingo@...e.hu
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be
 reused immediately.

On Thu, Jun 10, 2010 at 01:09:11PM -0700, Salman wrote:
> A program that repeatedly forks and waits is susceptible to having the
> same pid repeated, especially when it competes with another instance of the
> same program.  This is really bad for bash implementation.  Furthermore, many shell
> scripts assume that pid numbers will not be used for some length of time.

This should probably get wrapped at column 74 or so....

> +static int pid_before(int base, int a, int b)
> +{
> +	/*
> +	 * This is the same as saying
> +	 *
> +	 * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
> +	 * and that mapping orders 'a' and 'b' with respect to 'base'.
> +	 *
> +	 */
> +	return (unsigned)(a - base) < (unsigned)(b - base);
> +}

Does this work though if /proc/sys/kernel/pid_max is not set to
MAXUINT?

I like the optimization, but it looks like pid_max defaults to 4096 if
CONFIG_BASE_SMALL is set, and 32768 otherwise.

Am I missing something?

					- Ted
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