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

[Oleg Nesterov <oleg@redhat.com>]

On 03/16, Tejun Heo wrote:
>
> On Thu, Mar 16, 2017 at 11:33:01AM -0400, Tejun Heo wrote:
> > > 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.
>
> Oops, as for adding lockless_assign_pointer(), wouldn't smp_wmb() be a
> better match for smp_read_barrier_depends()?  ISTR acquire/release
> pairs being more expensive on some archs.

No, no, don't ask me, I can't know ;)

But. Note that rcu_assign_pointer() (which should pair with
smp_read_barrier_depends/lockless_dereference too) uses
smp_store_release(), and the changelog says "potentially less overhead".
See 88c1863066ccfa456 "rcu: Define rcu_assign_pointer() in terms of
smp_store_release()".

And this discussion is another argument to add the new helper,
we can always change it to use wmb or store_release, or whatever else.
Plus arch/ can overwrite it.

Oleg.