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

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Message-ID: <20130302172003.GC29769@redhat.com>
Date:	Sat, 2 Mar 2013 18:20:03 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Lai Jiangshan <eag0628@...il.com>, linux-doc@...r.kernel.org,
	peterz@...radead.org, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, namhyung@...nel.org,
	mingo@...nel.org, linux-arch@...r.kernel.org,
	linux@....linux.org.uk, xiaoguangrong@...ux.vnet.ibm.com,
	wangyun@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	rusty@...tcorp.com.au, rostedt@...dmis.org, rjw@...k.pl,
	vincent.guittot@...aro.org, tglx@...utronix.de,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	sbw@....edu, tj@...nel.org, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V2] lglock: add read-preference local-global rwlock

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.

Otherwise, at first glance looks correct...

However, I still think that FALLBACK_BASE only adds the unnecessary
complications. But even if I am right this is subjective of course, please
feel free to ignore.

And btw, I am not sure about lg->lock_dep_map, perhaps we should use
fallback_rwlock->dep_map ?

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.

Oleg.

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