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Date: Thu, 16 Mar 2017 16:46:15 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, Chris Mason <clm@fb.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 1/2] kthread: add barriers to set_kthread_struct() and
        to_kthread()
Message-ID: <20170316154614.GB26391@redhat.com>
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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.