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

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Date:	Fri, 3 Jun 2016 16:57:55 -0400
From:	Waiman Long <waiman.long@....com>
To:	xinhui <xinhui.pan@...ux.vnet.ibm.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 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().

Cheers,
Longman