linux-kernel - Re: [PATCH 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is

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Date:	Thu, 19 Jun 2014 09:58:16 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<cl@...ux-foundation.org>, <kmo@...erainc.com>,
	<linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 6/6] percpu-refcount: implement percpu_ref_reinit() and
 percpu_ref_is_zero()

On 06/18/2014 11:32 PM, Tejun Heo wrote:
> On Wed, Jun 18, 2014 at 11:37:35AM +0800, Lai Jiangshan wrote:
>>> @@ -97,7 +98,10 @@ static inline void percpu_ref_kill(struct percpu_ref *ref)
>>>  static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
>>>  				    unsigned __percpu **pcpu_countp)
>>>  {
>>> -	unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
>>> +	unsigned long pcpu_ptr;
>>> +
>>> +	/* paired with smp_store_release() in percpu_ref_reinit() */
>>> +	pcpu_ptr = smp_load_acquire(&ref->pcpu_count_ptr);
>>
>>
>> Does "smp_load_acquire()" hurts the performance of percpu_ref_get/put()
>> in non-x86 system?
> 
> It's equivalent to data dependency barrier.  The only arch which needs
> something more than barrier() is alpha.  It isn't an issue.
> 

But I searched from the source, smp_load_acquire() is just barrier() in
x86, arm64, ia64, s390, sparc, but it includes memory barrier 
instruction in other archs.

CC Paul. If smp_load_acquire() is sufficient lightweight, I would update
the SRCU.

Thanks,
Lai
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