linux-kernel - Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

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Message-ID: <CANN689EhqoQAKALR7WhT3YrQh3=VDA2Dq3QX1yLuYU_gZAo_Tg@mail.gmail.com>
Date:	Thu, 28 Feb 2013 19:34:31 +0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"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 v6 04/46] percpu_rwlock: Implement the core design of
 Per-CPU Reader-Writer Locks

On Thu, Feb 28, 2013 at 3:25 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 02/27, Michel Lespinasse wrote:
>>
>> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
>> +{
>> +       preempt_disable();
>> +
>> +       if (__this_cpu_read(*lgrw->local_refcnt) ||
>> +           arch_spin_trylock(this_cpu_ptr(lgrw->lglock->lock))) {
>> +               __this_cpu_inc(*lgrw->local_refcnt);
>
> Please look at __this_cpu_generic_to_op(). You need this_cpu_inc()
> to avoid the race with irs. The same for _read_unlock.

Hmmm, I was thinking that this was safe because while interrupts might
modify local_refcnt to acquire a nested read lock, they are expected
to release that lock as well which would set local_refcnt back to its
original value ???

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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