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

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Date:	Sat, 02 Mar 2013 22:41:18 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
CC:	Michel Lespinasse <walken@...gle.com>,
	Oleg Nesterov <oleg@...hat.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 03/02/2013 06:44 PM, Lai Jiangshan wrote:
> From 345a7a75c314ff567be48983e0892bc69c4452e7 Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@...fujitsu.com>
> Date: Sat, 2 Mar 2013 20:33:14 +0800
> Subject: [PATCH] lglock: add read-preference local-global rwlock
> 
> Current lglock is not read-preference, so it can't be used on some cases
> which read-preference rwlock can do. Example, get_cpu_online_atomic().
> 
[...]
> diff --git a/kernel/lglock.c b/kernel/lglock.c
> index 6535a66..52e9b2c 100644
> --- a/kernel/lglock.c
> +++ b/kernel/lglock.c
> @@ -87,3 +87,71 @@ void lg_global_unlock(struct lglock *lg)
>  	preempt_enable();
>  }
>  EXPORT_SYMBOL(lg_global_unlock);
> +
> +#define FALLBACK_BASE	(1UL << 30)
> +
> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
> +{
> +	struct lglock *lg = &lgrw->lglock;
> +
> +	preempt_disable();
> +	if (likely(!__this_cpu_read(*lgrw->reader_refcnt))) {
> +		rwlock_acquire_read(&lg->lock_dep_map, 0, 0, _RET_IP_);
> +		if (unlikely(!arch_spin_trylock(this_cpu_ptr(lg->lock)))) {
> +			read_lock(&lgrw->fallback_rwlock);
> +			__this_cpu_write(*lgrw->reader_refcnt, FALLBACK_BASE);
> +			return;
> +		}
> +	}
> +
> +	__this_cpu_inc(*lgrw->reader_refcnt);
> +}
> +EXPORT_SYMBOL(lg_rwlock_local_read_lock);
> +
> +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;

This should be a break, instead of a return, right?
Otherwise, there will be a preempt imbalance...

> +	case FALLBACK_BASE:
> +		__this_cpu_write(*lgrw->reader_refcnt, 0);
> +		read_unlock(&lgrw->fallback_rwlock);
> +		rwlock_release(&lg->lock_dep_map, 1, _RET_IP_);
> +		break;
> +	default:
> +		__this_cpu_dec(*lgrw->reader_refcnt);
> +		break;
> +	}
> +
> +	preempt_enable();
> +}


Regards,
Srivatsa S. Bhat

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