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

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Date:	Mon, 06 Jun 2016 11:15:01 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Waiman Long <waiman.long@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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04日 04:57, Waiman Long wrote:
> On 06/03/2016 03:17 AM, xinhui wrote:
>>
>> On 2016年06月02日 19:02, Peter Zijlstra wrote:
>>> On Thu, Jun 02, 2016 at 12:44:51PM +0200, 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, loads. And while this might be a suitable fix for asm-generic, it
>>> will introduce a fairly large regression on x86 (which is currently the
>>> only user of this).
>>>
>> well, to show respect to struct __qrwlock private field.
>> We can keep smp_store_release((u8 *)&lock->cnts, 0) in little_endian machine.
>> as this should be quick and no performance issue to all other archs(although there is only 1 now)
>>
>> BUT, We need use (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts) in big_endian machine.
>> because it's bad to export struct __qrwlock and set its private field to NULL.
>>
>> How about code like below.
>>
>> static inline void queued_write_unlock(struct qrwlock *lock)
>> {
>> #ifdef __BIG_ENDIAN
>>         (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
>> #else
>>     smp_store_release((u8 *)&lock->cnts, 0);
>> #endif
>> }
>>
>> BUT I think that would make thing a little complex to understand. :(
>> So at last, in my opinion, I suggest my patch :)
>> any thoughts?
>
> Another alternative is to make queued_write_unlock() overrideable from asm/qrwlock.h, just like what we did with queued_spin_unlock().
>
fair enough :)
And archs can write better code for themself.
I will send patch v2 with suggested-by of you. :)

thanks
xinhui

> Cheers,
> Longman
>