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

Tejun Heo
2017-03-16T15:33:07Z

Hello, Oleg.

On Thu, Mar 16, 2017 at 03:54:36PM +0100, Oleg Nesterov wrote:
> 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.

Yeah, I could have been clearer.

> > + /*
> > + * 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.

Ah yeah, that would look better. I vaguely remembered the new macro
but couldn't quite remember it fully. :) Will update the patch.

Thanks.

--
tejun