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

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Date:	Tue, 05 Mar 2013 23:41:48 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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 03/03/13 01:11, Srivatsa S. Bhat wrote:
> 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...


"lockdep" and "preempt" are handled in lg_local_unlock(&lgrw->lglock);

Thanks,
Lai

> 
>> +	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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