linux-kernel - Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian

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Date:	Fri, 03 Jun 2016 15:20:53 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, waiman.long@...com
Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian



On 2016年06月02日 19:15, Peter Zijlstra wrote:
> On Thu, Jun 02, 2016 at 07:01:17PM +0800, xinhui wrote:
>>
>> On 2016年06月02日 18:44, Arnd Bergmann wrote:
>>> On Thursday, June 2, 2016 6:09:08 PM CEST Pan Xinhui wrote:
>>>> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
>>>> index 54a8e65..eadd7a3 100644
>>>> --- a/include/asm-generic/qrwlock.h
>>>> +++ b/include/asm-generic/qrwlock.h
>>>> @@ -139,7 +139,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
>>>>    */
>>>>   static inline void queued_write_unlock(struct qrwlock *lock)
>>>>   {
>>>> -       smp_store_release((u8 *)&lock->cnts, 0);
>>>> +       (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
>>>>   }
>>>
>>> Isn't this more expensive than the existing version?
>>>
>> yes, a little more expensive than the existing version
>
> Think 20+ cycles worse.
>
>> But does this is generic code, I am not sure how it will impact the performance on other archs.
>
> As always, you get to audit users of stuff you change. And here you're
> lucky, there's only 1.
>
yes, and hope there will be 2 :)

>> If you like
>> we calculate the correct address to set to NULL
>> say,
>> static inline void queued_write_unlock(struct qrwlock *lock)
>> {
>> u8 *wl = lock;
>>
>> #ifdef __BIG_ENDIAN
>> wl += 3;
>> #endif
>> smp_store_release(wl, 0);
>>
>> }
>
> No, that's horrible. Either lift __qrwlock into qrwlock_types.h or do
> what qspinlock does. And looking at that, we could make
agree.

> queued_spin_unlock() use the atomic_sub_return_relaxed() thing too I
> suppose, that generates slightly better code.
>
thanks for your suggestion.
I can have a try in queued_spin_unlock().