Re: [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock_sched

[Oleg Nesterov <oleg@redhat.com>]

On 10/24, Paul E. McKenney wrote:
>
> On Wed, Oct 24, 2012 at 07:18:55PM +0200, Oleg Nesterov wrote:
> > On 10/24, Paul E. McKenney wrote:
> > >
> > > static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> > > {
> > > 	/*
> > > 	 * Decrement our count, but protected by RCU-sched so that
> > > 	 * the writer can force proper serialization.
> > > 	 */
> > > 	rcu_read_lock_sched();
> > > 	this_cpu_dec(*p->counters);
> > > 	rcu_read_unlock_sched();
> > > }
> >
> > Yes, the explicit lock/unlock makes the new assumptions about
> > synchronize_sched && barriers unnecessary. And iiuc this could
> > even written as
> >
> > 	rcu_read_lock_sched();
> > 	rcu_read_unlock_sched();
> >
> > 	this_cpu_dec(*p->counters);
>
> But this would lose the memory barrier that is inserted by
> synchronize_sched() after the CPU's last RCU-sched read-side critical
> section.

How? Afaics there is no need to synchronize with this_cpu_dec(), its
result was already seen before the 2nd synchronize_sched() was called
in percpu_down_write().

IOW, this memory barrier is only needed to synchronize with memory
changes inside down_read/up_read.

To clarify, of course I do not suggest to write is this way. I am just
trying to check my understanding.

Oleg.