Re: [RFC][PATCH 6/9] seqlock: Better document raw_write_seqcount_latch()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Mar 02, 2015 at 09:51:16AM +0100, Peter Zijlstra wrote:
> On Mon, Mar 02, 2015 at 09:33:20AM +0100, Peter Zijlstra wrote:
> > On Sun, Mar 01, 2015 at 02:02:23PM +0000, Mathieu Desnoyers wrote:
> 
> > > The latch, AFAIU, takes care of making sure the new objects are
> > > initialized before being published into the data structure, so there
> > > would be no need to use RCU assign pointer. However, we really need
> > > RCU around reads, along with a grace period between removal of an object
> > > and its teardown.
> > 
> > So I do need the rcu_assign_pointer for the RB link because that also
> > initializes the rb_node itself. Or put differently, be _very_ _VERY_
> > sure your entire object is initialized before the latch.
> > 
> > Secondly, note that the latch does a WMB and rcu_assign_pointer does a
> > RELEASE, these are not equivalent.
> > 
> > So I don't think I will highlight this particular point. If you're sure
> > enough to know the difference you can get away with it, sure. But in
> > general I think people should still use rcu_assign_pointer; if only to
> > make Paul sleep better at night ;-)

I do appreciate that sentiment.  ;-)

> Also note that if you do not use rcu_assign_pointer() one will at the
> very least require WRITE_ONCE().

And, for ARM, IA64, and PowerPC, memory-barrier instructions.

							Thanx, Paul