Re: [PATCH 1/2] kthread: add barriers to set_kthread_struct() and to_kthread()

Oleg Nesterov
2017-03-16T14:58:11Z

Hi Tejun,

On 03/15, Tejun Heo wrote:
>
> Until now, all to_kthread() users are interlocked with kthread
> creation and there's no need to have explicit barriers when setting
> the kthread pointer or dereferencing it.
>
> However, There is a race condition where userland can interfere with a
> kthread while it's being initialized. To close it, to_kthread() needs
> to be used from an unsynchronized context.

So this is preparation for 2/2... IIUC, the current code is not buggy,
just you need to add kthread_initialized() which can't work without
this change.

> static inline void set_kthread_struct(void *kthread)
> {
> + /* paired with smp_read_data_barrier_depends() in to_kthread() */
> + smp_wmb();
> +
> /*
> * We abuse ->set_child_tid to avoid the new member and because it
> * can't be wrongly copied by copy_process(). We also rely on fact
> @@ -67,8 +70,19 @@ static inline void set_kthread_struct(vo
>
> static inline struct kthread *to_kthread(struct task_struct *k)
> {
> + void *ptr;
> +
> WARN_ON(!(k->flags & PF_KTHREAD));
> - return (__force void *)k->set_child_tid;
> +
> + ptr = (__force void *)k->set_child_tid;
> +
> + /*
> + * Paired with smp_wmb() in set_kthread_struct() and ensures that
> + * the caller sees initialized content of the returned kthread.
> + */
> + smp_read_barrier_depends();
> +
> + return ptr;

This is almost off-topic, but I think lockless_dereference() will look
better in to_kthread().

And perhaps we should add another helper, say,

#define lockless_assign_pointer(ptr, val) \
smp_store_release(&ptr, val)

for set_kthread_struct() ? it can have more users.

Not that I think you should change your patch, I am just asking.

Oleg.