[PATCH review 2/3] pidns: Stop pid allocation when init dies

[Oleg Nesterov]

On 12/21, Eric W. Biederman wrote:
>
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -21,7 +21,7 @@ struct pid_namespace {
>  	struct kref kref;
>  	struct pidmap pidmap[PIDMAP_ENTRIES];
>  	int last_pid;
> -	int nr_hashed;
> +	unsigned int nr_hashed;
>  	struct task_struct *child_reaper;
>  	struct kmem_cache *pid_cachep;
>  	unsigned int level;
> @@ -42,6 +42,8 @@ struct pid_namespace {
>
>  extern struct pid_namespace init_pid_ns;
>
> +#define PIDNS_HASH_ADDING (1U << 31)

Yes, agreed. We can't rely on PF_EXITING/whatever, we need the explicit
flag.

1/2 looks fine too. Only one nit about init_pid_ns below...


> @@ -319,7 +318,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>
>  	upid = pid->numbers + ns->level;
>  	spin_lock_irq(&pidmap_lock);
> -	if (ns->nr_hashed < 0)
> +	if (ns->nr_hashed < PIDNS_HASH_ADDING)

I won't insist, but perhaps if "(!(nr_hashed & PIDNS_HASH_ADDING))"
looks more understandable.

> +void disable_pid_allocation(struct pid_namespace *ns)
> +{
> +	spin_lock_irq(&pidmap_lock);
> +	if (ns->nr_hashed >= PIDNS_HASH_ADDING)

Do we really need this check? It seems that PIDNS_HASH_ADDING
bit must be always set when disable_pid_allocation() is called.

> +		ns->nr_hashed -= PIDNS_HASH_ADDING;

Anyway, nr_hashed &= ~PIDNS_HASH_ADDING looks simpler and doesn't
need a check.

But again, I won't insist this is minor and subjective.

>  struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
>  {
>  	struct hlist_node *elem;
> @@ -584,7 +591,7 @@ void __init pidmap_init(void)
>  	/* Reserve PID 0. We never call free_pidmap(0) */
>  	set_bit(0, init_pid_ns.pidmap[0].page);
>  	atomic_dec(&init_pid_ns.pidmap[0].nr_free);
> -	init_pid_ns.nr_hashed = 1;
> +	init_pid_ns.nr_hashed = 1 + PIDNS_HASH_ADDING;

The obly chunk which doesn't look exactly correct to me, although this
doesn't really matter. Hmm, actually the code was already wrong before
this patch.

I think init_pid_ns.nr_hashed should be PIDNS_HASH_ADDING, we should not
add 1 to account the unused zero pid, and kernel_thread(kernel_init) was
not called yet.

Oleg.