On Wed, Jul 15, 2015 at 08:15:11PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 15, 2015 at 11:05:46AM -0700, Paul E. McKenney wrote:
> > On Sun, Jul 12, 2015 at 01:35:48AM +0200, Oleg Nesterov wrote:
> > > It is functionally equivalent to
> > >
> > > struct rcu_sync_struct {
> > > atomic_t counter;
> > > };
> > >
> > > static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
> > > {
> >
> > If you add an smp_mb() here...
> >
> > > return atomic_read(&rss->counter) == 0;
> > > }
> > >
> > > static inline void rcu_sync_enter(struct rcu_sync_struct *rss)
> > > {
> > > atomic_inc(&rss->counter);
> > > synchronize_sched();
> > > }
> > >
> > > static inline void rcu_sync_exit(struct rcu_sync_struct *rss)
> > > {
> > > synchronize_sched();
> >
> > You should be able to demote the above synchronize_sched() to an
> > smp_mb__before_atomic(). Even rare writes should make this tradeoff
> > worthwhile.
>
> No, it makes the read-side primitive contain an unconditional memory
> barrier, that forgoes the entire point.
>
> The writers are stupidly expensive already for they need global
> serialization, optimizing them in any way doesn't make sense.
That could well be the case, but it would be good to see the numbers.
Thanx, Paul
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