linux-kernel - Re: [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin

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Date:	Fri, 27 May 2016 18:34:58 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	virtualization@...ts.linux-foundation.org,
	benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
	mingo@...hat.com, paulmck@...ux.vnet.ibm.com, jeremy@...p.org,
	chrisw@...s-sol.org, akataria@...are.com, rusty@...tcorp.com.au
Subject: Re: [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin_unlock



On 2016年05月27日 00:57, Peter Zijlstra wrote:
> On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra wrote:
>> On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote:
>>> cmpxchg_release is light-wight than cmpxchg, we can gain a better
>>> performace then. On some arch like ppc, barrier impact the performace
>>> too much.
>>>
>>> Suggested-by:  Boqun Feng <boqun.feng@...il.com>
>>> Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
>>> ---
>>>   kernel/locking/qspinlock_paravirt.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
>>> index a5b1248..2bbffe4 100644
>>> --- a/kernel/locking/qspinlock_paravirt.h
>>> +++ b/kernel/locking/qspinlock_paravirt.h
>>> @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
>>>   	 * unhash. Otherwise it would be possible to have multiple @lock
>>>   	 * entries, which would be BAD.
>>>   	 */
>>> -	locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
>>> +	locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
>>>   	if (likely(locked == _Q_LOCKED_VAL))
>>>   		return;
>>
>> This patch fails to explain _why_ it can be relaxed.
>>
>> And seeing how this cmpxchg() can actually unlock the lock, I don't see
>> how this can possibly be correct. Maybe cmpxchg_release(), but relaxed
>> seems very wrong.
>
> Clearly I need to stop working for the day, I cannea read. You're doing
> release, not relaxed.
>
Never mind.  thanks for review :)

> Still Changelog needs improvement.
>
Will do that.

thanks
xinhui