Re: [RFC] introduce synchronize_sched_{enter,exit}()

[Peter Zijlstra]

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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