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

Ingo Molnar
2010-06-09T09:49:16Z

* Salman <sqazi@google.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@google.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