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

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Date:	Wed, 9 Jun 2010 11:48:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Salman <sqazi@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	peterz@...radead.org, akpm@...x-foundation.org,
	linux-kernel@...r.kernel.org, tytso@...gle.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be
 reused immediately.


* Salman <sqazi@...gle.com> 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.
> 
> Thanks to Ted Tso for the key ideas of this implementation.
> 
> Signed-off-by: Salman Qazi <sqazi@...gle.com>
> ---
>  kernel/pid.c |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index e9fd8c1..8cedeab 100644
> --- a/kernel/pid.c
> +++ 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);
> +						if (last >= pid)
> +							break;
> +					} while (prev != last);
> +
>  					return pid;
>  				}
>  				offset = find_next_offset(map, offset);

Looks rather interesting. (Cleanliness-wise i'd suggest to split out the while 
loop into a helper function.)

	Ingo
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