linux-kernel - Re: [PATCH 4/6] arch/sparc: Enable queued rwlocks for SPARC

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Date:   Fri, 19 May 2017 11:43:10 -0500
From:   Babu Moger <babu.moger@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        David Miller <davem@...emloft.net>
Cc:     mingo@...hat.com, arnd@...db.de, shannon.nelson@...cle.com,
        haakon.bugge@...cle.com, steven.sistare@...cle.com,
        vijay.ac.kumar@...cle.com, jane.chu@...cle.com,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH 4/6] arch/sparc: Enable queued rwlocks for SPARC


On 5/19/2017 4:03 AM, Peter Zijlstra wrote:
> On Thu, May 18, 2017 at 10:31:13PM -0400, David Miller wrote:
>> From: Babu Moger <babu.moger@...cle.com>
>> Date: Thu, 18 May 2017 18:36:08 -0600
>>
>>> @@ -82,6 +82,7 @@ config SPARC64
>>>   	select HAVE_ARCH_AUDITSYSCALL
>>>   	select ARCH_SUPPORTS_ATOMIC_RMW
>>>   	select HAVE_NMI
>>> +	select ARCH_USE_QUEUED_RWLOCKS
>>>   
>> If you are selecting this on SPARC64 all the time, then:
>>
>>> @@ -94,6 +94,7 @@ static inline void arch_spin_lock_flags(arch_spinlock_t *lock, unsigned long fla
>>>   	: "memory");
>>>   }
>>>   
>>> +#ifndef CONFIG_QUEUED_RWLOCKS
>>>   /* Multi-reader locks, these are much saner than the 32-bit Sparc ones... */
>> You can remove this segment of ifdef'd code altogether since it is in
>> a sparc64 specific header file.
>
> So IIRC Sparc v8 only has that single byte load-and-set (or swap)
> instruction, right? That means you can only make test-and-set spinlocks
> and then have to build the world on top of that.
>
> I don't see qrwlock -- which assumes the spinlock implementation is fair
> -- making much sense for that.
>
> Also, IIRC Sparc-v8 didn't really have very big SMP systems, those came
> with v9. And qspinlock only really makes sense on the bigger systems
> (not to mention that building the qspinlock on top of atomic operations
> build on test-and-set spinlocks just seems extremely dysfunctional).
>
>
> In any case, I think what I'm saying is that it makes sense to make this
> a Sparcv9 only feature.
>

Agree.  Lets keep this as Sparcv9 only feature.