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().
For the kfree_rcu() performance testing I was working on recently, replacing
xchg() with a pair of assignment statements in the code being tested provided
a great performance increase (on x86).
thanks,
- Joel
