Re: [PATCH tip/core/rcu 1/9] rcu: Upgrade rcu_swap_protected() to rcu_replace()

[Peter Zijlstra]

On Thu, Oct 03, 2019 at 06:33:15AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 03, 2019 at 09:08:50AM -0400, Steven Rostedt wrote:
> > On Thu, 03 Oct 2019 09:39:17 +0100
> > David Howells <dhowells@xxxxxxxxxx> wrote:
> > 
> > > paulmck@xxxxxxxxxx wrote:
> > > 
> > > > +#define rcu_replace(rcu_ptr, ptr, c)					\
> > > > +({									\
> > > > +	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
> > > > +	rcu_assign_pointer((rcu_ptr), (ptr));				\
> > > > +	__tmp;								\
> > > > +})  
> > > 
> > > Does it make sense to actually use xchg() if that's supported by the arch?
> 
> Historically, xchg() has been quite a bit slower than a pair of assignment
> statements, in part due to the strong memory ordering guaranteed by
> xchg().  Has that changed?  If so, then, agreed, it might make sense to
> use xchg().

Nope, still the case. xchg() is an atomic op with full ordering.