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

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Message-Id: <575011FD.4070109@linux.vnet.ibm.com>
Date:	Thu, 02 Jun 2016 19:01:17 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	waiman.long@...com, peterz@...radead.org
Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian


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
But does this is generic code, I am not sure how it will impact the performance on other archs.

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);

}



> 	Arnd
>