linux-kernel - Re: [PATCH v2 06/28] locking/rwlocks: Add contention detection for rwlocks

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Date:   Tue, 9 Feb 2021 19:27:52 -0500
From:   Waiman Long <longman@...hat.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH v2 06/28] locking/rwlocks: Add contention detection for
 rwlocks

On 2/9/21 5:25 PM, Guenter Roeck wrote:
> On Tue, Feb 09, 2021 at 04:46:02PM -0500, Waiman Long wrote:
>> On 2/9/21 3:39 PM, Guenter Roeck wrote:
>>> On Tue, Feb 02, 2021 at 10:57:12AM -0800, Ben Gardon wrote:
>>>> rwlocks do not currently have any facility to detect contention
>>>> like spinlocks do. In order to allow users of rwlocks to better manage
>>>> latency, add contention detection for queued rwlocks.
>>>>
>>>> CC: Ingo Molnar <mingo@...hat.com>
>>>> CC: Will Deacon <will@...nel.org>
>>>> Acked-by: Peter Zijlstra <peterz@...radead.org>
>>>> Acked-by: Davidlohr Bueso <dbueso@...e.de>
>>>> Acked-by: Waiman Long <longman@...hat.com>
>>>> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
>>>> Signed-off-by: Ben Gardon <bgardon@...gle.com>
>>> When building mips:defconfig, this patch results in:
>>>
>>> Error log:
>>> In file included from include/linux/spinlock.h:90,
>>>                    from include/linux/ipc.h:5,
>>>                    from include/uapi/linux/sem.h:5,
>>>                    from include/linux/sem.h:5,
>>>                    from include/linux/compat.h:14,
>>>                    from arch/mips/kernel/asm-offsets.c:12:
>>> arch/mips/include/asm/spinlock.h:17:28: error: redefinition of 'queued_spin_unlock'
>>>      17 | #define queued_spin_unlock queued_spin_unlock
>>>         |                            ^~~~~~~~~~~~~~~~~~
>>> arch/mips/include/asm/spinlock.h:22:20: note: in expansion of macro 'queued_spin_unlock'
>>>      22 | static inline void queued_spin_unlock(struct qspinlock *lock)
>>>         |                    ^~~~~~~~~~~~~~~~~~
>>> In file included from include/asm-generic/qrwlock.h:17,
>>>                    from ./arch/mips/include/generated/asm/qrwlock.h:1,
>>>                    from arch/mips/include/asm/spinlock.h:13,
>>>                    from include/linux/spinlock.h:90,
>>>                    from include/linux/ipc.h:5,
>>>                    from include/uapi/linux/sem.h:5,
>>>                    from include/linux/sem.h:5,
>>>                    from include/linux/compat.h:14,
>>>                    from arch/mips/kernel/asm-offsets.c:12:
>>> include/asm-generic/qspinlock.h:94:29: note: previous definition of 'queued_spin_unlock' was here
>>>      94 | static __always_inline void queued_spin_unlock(struct qspinlock *lock)
>>>         |                             ^~~~~~~~~~~~~~~~~~
>> I think the compile error is caused by the improper header file inclusion
>> ordering. Can you try the following change to see if it can fix the compile
>> error?
>>
> That results in:
>
> In file included from ./arch/mips/include/generated/asm/qrwlock.h:1,
>                   from ./arch/mips/include/asm/spinlock.h:13,
>                   from ./include/linux/spinlock.h:90,
>                   from ./include/linux/ipc.h:5,
>                   from ./include/uapi/linux/sem.h:5,
>                   from ./include/linux/sem.h:5,
>                   from ./include/linux/compat.h:14,
>                   from arch/mips/kernel/asm-offsets.c:12:
> ./include/asm-generic/qrwlock.h: In function 'queued_rwlock_is_contended':
> ./include/asm-generic/qrwlock.h:127:9: error: implicit declaration of function 'arch_spin_is_locked'
>
> Guenter

It is because in arch/mips/include/asm/spinlock.h, asm/qrwlock.h is 
included before asm/qspinlock.h. The compilation error should be gone if 
the asm/qrwlock.h is removed or moved after asm/qspinlock.h.

I did a x86 build and there was no compilation issue.

Cheers,
Longman