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

From	xinhui <xinhui.pan@linux.vnet.ibm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian
Date	2016-06-03 09:30 +0200
Message-ID	<rFRIl-cH-7@gated-at.bofh.it> (permalink)
References	<rFxJE-4D3-35@gated-at.bofh.it> <rFyml-4Qa-9@gated-at.bofh.it> <rFyFI-5bR-13@gated-at.bofh.it> <rFyPo-5f5-5@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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().

Thread

[PATCH] locking/qrwlock: fix write unlock issue in big endian Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> - 2016-06-02 12:10 +0200
  Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian Arnd Bergmann <arnd@arndb.de> - 2016-06-02 12:50 +0200
    Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian xinhui <xinhui.pan@linux.vnet.ibm.com> - 2016-06-02 13:10 +0200
      Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian Peter Zijlstra <peterz@infradead.org> - 2016-06-02 13:20 +0200
        Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian xinhui <xinhui.pan@linux.vnet.ibm.com> - 2016-06-03 09:30 +0200
    Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian Peter Zijlstra <peterz@infradead.org> - 2016-06-02 13:10 +0200
      Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian xinhui <xinhui.pan@linux.vnet.ibm.com> - 2016-06-03 09:20 +0200
      Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian xinhui <xinhui.pan@linux.vnet.ibm.com> - 2016-06-06 05:20 +0200
  Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian Will Deacon <will.deacon@arm.com> - 2016-06-08 11:30 +0200

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