Re: [RFC] Race condition?

[Oliver Neukum]

Am Freitag, 2. August 2002 19:00 schrieb Dave Hansen:
> Kasper Dupont wrote:
> > Is there a race condition in this piece of code from do_fork in
> > linux/kernel/fork.c? I cannot see what prevents two processes
> > from calling this at the same time and both successfully fork
> > even though the user had only one process left.
> >
> >         if (atomic_read(&p->user->processes) >=
> > p->rlim[RLIMIT_NPROC].rlim_cur && !capable(CAP_SYS_ADMIN) &&
> > !capable(CAP_SYS_RESOURCE)) goto bad_fork_free;
> >
> >         atomic_inc(&p->user->__count);
> >         atomic_inc(&p->user->processes);
>
> I don't see any locking in the call chain leading to this function, so
> I think you're right.  The attached patch fixes this.  It costs an
> extra 2 atomic ops in the failure case, but otherwise just makes the
> processes++ operation earlier.
>
> Patch is against 2.5.27, but applies against 30.

It has the opposite failure mode. Forks only some of which should
succeed may all fail.

	Regards
		Oliver