Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Applyrcu_access_pointer() to avoid sparse false positive

[Josh Triplett]

On Wed, Oct 09, 2013 at 05:28:33PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 09, 2013 at 05:12:40PM -0700, Eric Dumazet wrote:
> > On Wed, 2013-10-09 at 16:40 -0700, Josh Triplett wrote:
> > 
> > > that.  Constructs like list_del_rcu are much clearer, and not
> > > open-coded.  Open-coding synchronization code is almost always a Bad
> > > Idea.
> > 
> > OK, so you think there is synchronization code.
> > 
> > I will shut up then, no need to waste time.
> 
> As you said earlier, we should at least get rid of the memory barrier
> as long as we are changing the code.
> 
> Josh, what would you suggest as the best way to avoid the memory barrier,
> keep sparse happy, and not be too ugly?

The more I think about it, the more I realize that assigning an __rcu
pointer to an __rcu pointer *without* a memory barrier is a sufficiently
uncommon case that you probably *should* just write an open-coded
assignment.  Just please put a very clear comment right before it.

I'd originally thought it might make sense to have a macro similar to
rcu_assign_pointer, but I just don't think this is a common enough case,
and we don't want people thinking they can use this in general for __rcu
to __rcu assignments (most of which still need a memory barrier).

- Josh Triplett
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