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

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Message-ID: <87d3w066j8.fsf@basil.nowhere.org>
Date:	Wed, 09 Jun 2010 08:53:31 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Salman <sqazi@...gle.com>
Cc:	peterz@...radead.org, mingo@...e.hu, akpm@...x-foundation.org,
	linux-kernel@...r.kernel.org, tytso@...gle.com
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

Salman <sqazi@...gle.com> writes:
> +++ b/kernel/pid.c
> @@ -153,8 +153,17 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
>  		if (likely(atomic_read(&map->nr_free))) {
>  			do {
>  				if (!test_and_set_bit(offset, map->page)) {
> +					int prev;
>  					atomic_dec(&map->nr_free);
> -					pid_ns->last_pid = pid;
> +
> +					do {
> +						prev = last;
> +						last = cmpxchg(&pid_ns->last_pid,
> +							       prev,
> +						pid);

At some point not all architectures in Linux supported cmpxchg,
so it was not allowed to use it unconditionally in portable code.

This might have changed now (at least the UP only architectures fall
back to a generic cmpxchg now I think), but I'm not sure you have full
coverage on SMP.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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