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

From: [email-masked] (Oleg Nesterov)
Date: Fri, 1 Mar 2013 19:28:54 +0100

Lai, I didn't read this discussion except the code posted by Michel.
I'll try to read this patch carefully later, but I'd like to ask
a couple of questions.

This version looks more complex than Michel's, why? Just curious, I
am trying to understand what I missed. See
http://marc.info/?l=linux-kernel&m=136196350213593

And I can't understand FALLBACK_BASE...

OK, suppose that CPU_0 does _write_unlock() and releases ->fallback_rwlock.

CPU_1 does _read_lock(), and ...

> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
> +{
> +	struct lglock *lg = &lgrw->lglock;
> +
> +	preempt_disable();
> +	rwlock_acquire_read(&lg->lock_dep_map, 0, 0, _RET_IP_);
> +	if (likely(!__this_cpu_read(*lgrw->reader_refcnt))) {
> +		if (!arch_spin_trylock(this_cpu_ptr(lg->lock))) {

_trylock() fails,

> +			read_lock(&lgrw->fallback_rwlock);
> +			__this_cpu_add(*lgrw->reader_refcnt, FALLBACK_BASE);

so we take ->fallback_rwlock and ->reader_refcnt == FALLBACK_BASE.

CPU_0 does lg_global_unlock(lgrw->lglock) and finishes _write_unlock().

Interrupt handler on CPU_1 does _read_lock() notices ->reader_refcnt != 0
and simply does this_cpu_inc(), so reader_refcnt == FALLBACK_BASE + 1.

Then irq does _read_unlock(), and

> +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw)
> +{
> +	switch (__this_cpu_dec_return(*lgrw->reader_refcnt)) {
> +	case 0:
> +		lg_local_unlock(&lgrw->lglock);
> +		return;
> +	case FALLBACK_BASE:
> +		__this_cpu_sub(*lgrw->reader_refcnt, FALLBACK_BASE);
> +		read_unlock(&lgrw->fallback_rwlock);

hits this case?

Doesn't look right, but most probably I missed something.

Oleg.