On 07/15, 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...
I don't think so, please see below...
> > 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.
This is irrelevant I think, this (pseudo) code just tries to explain
what this interface does.
> > +static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
> > +{
>
> smp_mb(); /* A: Ensure that reader sees last update. */
> /* Pairs with B. */
>
Let me remind you about your f0a0e6f282c72247e7c8ec "rcu: Clarify
memory-ordering properties of grace-period primitives" documentation
patch ;)
We do not need any barrier, assuming that this is called under
preempt_disable/etc.
rcu_sync_is_idle() becomes true after another gp pass. The reader
should see all updates after that.
> > +void rcu_sync_exit(struct rcu_sync_struct *rss)
> > +{
> > + spin_lock_irq(&rss->rss_lock);
>
> smp_mb(); /* B: Make sure next readers see critical section. */
> /* Pairs with A. */
>
> > + if (!--rss->gp_count) {
>
> At which point, I believe you can ditch the callback entirely, along
> with ->cb_state.
>
> So, what am I missing here?
Please see above. We start anothe gp before "unlock" to avoid mb's in
the reader's code.
> Are readers really so frequent that the
> added read-side memory barrier is visible?
But this code is heavily optimized for the readers. And please see
another discussion about sb_writers and percpu_rw_semaphore. I was
suprized, but mb() in sb_start_write() is actually noticeable.
> Given that the current
> code forces the readers to grab ->rss_lock
Where? the readers never take this lock.
Oleg.
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