Re: [PATCH 2/3] rcu: Create rcu_sync infrastructure

[Oleg Nesterov <oleg@redhat.com>]

On 10/04, Peter Zijlstra wrote:
>
> On Fri, Oct 04, 2013 at 01:15:13PM +0200, Oleg Nesterov wrote:
> > > What's exclusive to mean? One writer at a time?
> >
> > Yes,
>
> I'm not entirely sure what the advantage is of having that logic in this
> primitive. Shouldn't that be something the user of this rcu_sync stuff
> does (or not) depending on its needs.

Yes, the user can do the locking itself. But I think this option can help.
If nothing else it can help to avoid another mutex/whatever and unnecessary
wakeup/scheule's, even if this is minor.

And. rcu_sync_enter() should be "bool", it should return "need_sync". IOW,
rcu_sync_enter() == T means that this thread has done the FAST -> SLOW
transition, this is particularly useful in "exclusive" mode.

Consider percpu_down_write(). It takes rw_sem for writing (and this blocks
the readers) before clear_fast_ctr(), but we only need to do this this
after sync_sched(), so it could do

	if (rcu_sync_enter(&brw->rcu_sync))
		atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
	else
		; /* the above was already done */

	/* exclude readers */
	down_write(&brw->rw_sem);

and now ->rw_sem is only needed to serialize readers/writer.

Sure, this all is minor (and we will probably copy the "pending writer"
logic from cpu_hotplug_begin/get_online_cpus).

But we can get this feature almost for free, so I think it makes sense.

Oleg.