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

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Date:	Sat, 02 Mar 2013 21:42:15 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>, paulmck@...ux.vnet.ibm.com
CC:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Lai Jiangshan <eag0628@...il.com>,
	Michel Lespinasse <walken@...gle.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, 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] lglock: add read-preference local-global rwlock

On 02/03/13 02:28, Oleg Nesterov wrote:
> 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

Michel changed my old draft version a little, his version is good enough for me.
My new version tries to add a little better nestable support with only
adding single __this_cpu_op() in _read_[un]lock().

> 
> 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.

Your are right, I just realized that I had spit a code which should be atomic.

I hope this patch(V2) can get more reviews.

My first and many locking knowledge is learned from Paul.
Paul, would you also review it?

Thanks,
Lai

> 
> Oleg.
> 
> --
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