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

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Message-ID: <51363179.2030109@linux.vnet.ibm.com>
Date:	Tue, 05 Mar 2013 23:25:05 +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 03/05/2013 09:11 PM, Lai Jiangshan wrote:
> 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);
> 

Ah, ok.. I had missed that.

Regards,
Srivatsa S. Bhat

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