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

tytso
2010-06-10T20:38:23Z

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