Subject: [PATCH V2] lglock: add read-preference local-global rwlock

From: [email-masked] (Oleg Nesterov)
Date: Tue, 5 Mar 2013 17:41:06 +0100

On 03/05, Lai Jiangshan wrote:
>
> On 03/03/13 01:20, Oleg Nesterov wrote:
> > On 03/02, Lai Jiangshan wrote:
> >>
> >> +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw)
> >> +{
> >> +	switch (__this_cpu_read(*lgrw->reader_refcnt)) {
> >> +	case 1:
> >> +		__this_cpu_write(*lgrw->reader_refcnt, 0);
> >> +		lg_local_unlock(&lgrw->lglock);
> >> +		return;
> >> +	case FALLBACK_BASE:
> >> +		__this_cpu_write(*lgrw->reader_refcnt, 0);
> >> +		read_unlock(&lgrw->fallback_rwlock);
> >> +		rwlock_release(&lg->lock_dep_map, 1, _RET_IP_);
> >
> > I guess "case 1:" should do rwlock_release() too.
>
> Already do it in "lg_local_unlock(&lgrw->lglock);" before it returns.
> (I like reuse old code)

Yes, I was wrong thanks. Another case when I didn't notice that you
re-use the regular lg_ code...

> > We need rwlock_acquire_read() even in the fast-path, and this acquire_read
> > should be paired with rwlock_acquire() in _write_lock(), but it does
> > spin_acquire(lg->lock_dep_map). Yes, currently this is the same (afaics)
> > but perhaps fallback_rwlock->dep_map would be more clean.
>
> I can't tell which one is better. I try to use fallback_rwlock->dep_map later.

I am not sure which one should be better too, please check.

Again, I forgot that _write_lock/unlock use lg_global_*() code.

Oleg.