Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE

[Davidlohr Bueso]

On Mon, 23 May 2016, Jason Low wrote:

> On Sat, 2016-05-21 at 09:04 -0700, Peter Hurley wrote:
> > On 05/18/2016 12:58 PM, Jason Low wrote:
> > > It should be fine to use the standard READ_ONCE here, even if it's just
> > > for documentation, as it's probably not going to cost anything in
> > > practice. It would be better to avoid adding any special macros for this
> > > which may just add more complexity.
> >
> > See, I don't understand this line of reasoning at all.
> >
> > I read this as "it's ok to be non-optimal here where were spinning CPU
> > time but not ok to be non-optimal generally elsewhere where it's
> > way less important like at init time".
>
> So I think there is a difference between using it during init time and
> using it here where we're spinning. During init time, initializing the
> owner field locklessly is normal. No other thread should be concurrently
> be writing to the field, since the structure is just getting
> initialized, so there are no surprises there.
>
> Our access of the owner field in this function is special in that we're
> using a bit of "lockless magic" to read and write to a field that gets
> concurrently accessed without any serialization. Since we're not taking
> the wait_lock in a scenario where we'd normally would take a lock, it
> would be good to have this documented.
>
> > And by the way, it's not just "here" but _everywhere_.
> > What about reading ->on_cpu locklessly?
>
> Sure, we could also use READ_ONCE when reading ->on_cpu  :)

Locking wise we should be covered with ->on_cpu as we're always under rcu_read_lock
(barrier, preempt_disable). But I'm not sure if this rule applies throughout the
scheduler, however, like it does in, say thread_group_cputime(). cpu_clock_sample()
(from posix timers) seems to mix and match being done under rcu. So ultimately I
think you're right.

Thanks,
Davidlohr