On Mon, Sep 30, 2013 at 02:59:42PM +0200, Peter Zijlstra wrote:
> >
> > static void cb_rcu_func(struct rcu_head *rcu)
> > {
> > struct xxx_struct *xxx = container_of(rcu, struct xxx_struct, cb_head);
> > long flags;
> >
> > BUG_ON(xxx->gp_state != GP_PASSED);
> > BUG_ON(xxx->cb_state == CB_IDLE);
> >
> > spin_lock_irqsave(&xxx->xxx_lock, flags);
> > if (xxx->gp_count) {
> > xxx->cb_state = CB_IDLE;
>
> This seems to be when a new xxx_begin() has happened after our last
> xxx_end() and the sync_sched() from xxx_begin() merges with the
> xxx_end() one and we're done.
>
> > } else if (xxx->cb_state == CB_REPLAY) {
> > xxx->cb_state = CB_PENDING;
> > call_rcu_sched(&xxx->cb_head, cb_rcu_func);
>
> A later xxx_exit() has happened, and we need to requeue to catch a later
> GP.
>
> > } else {
> > xxx->cb_state = CB_IDLE;
> > xxx->gp_state = GP_IDLE;
>
> Nothing fancy happened and we're done.
>
> > }
> > spin_unlock_irqrestore(&xxx->xxx_lock, flags);
> > }
> >
> > void xxx_exit(struct xxx_struct *xxx)
> > {
> > spin_lock_irq(&xxx->xxx_lock);
> > if (!--xxx->gp_count) {
> > if (xxx->cb_state == CB_IDLE) {
> > xxx->cb_state = CB_PENDING;
> > call_rcu_sched(&xxx->cb_head, cb_rcu_func);
> > } else if (xxx->cb_state == CB_PENDING) {
> > xxx->cb_state = CB_REPLAY;
> > }
> > }
> > spin_unlock_irq(&xxx->xxx_lock);
> > }
>
> So I don't immediately see the point of the concurrent write side;
> percpu_rwsem wouldn't allow this and afaict neither would
> freeze_super().
>
> Other than that; yes this makes sense if you care about write side
> performance and I think its solid.
Hmm, wait. I don't see how this is equivalent to:
xxx_end()
{
synchronize_sched();
atomic_dec(&xxx->counter);
}
For that we'd have to decrement xxx->gp_count from cb_rcu_func(),
wouldn't we?
Without that there's no guarantee the fast path readers will have a MB
to observe the write critical section, unless I'm completely missing
something obviuos here.
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