linux-kernel - Re: [PATCH V5] riscv: Add qspinlock support

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Message-ID: <f6513b4f-ef62-fd40-b031-27c9b7f57e00@redhat.com>
Date:   Mon, 20 Jun 2022 17:16:15 -0400
From:   Waiman Long <longman@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>, Guo Ren <guoren@...nel.org>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V5] riscv: Add qspinlock support


On 6/20/22 16:42, Arnd Bergmann wrote:
> On Mon, Jun 20, 2022 at 5:54 PM <guoren@...nel.org> wrote:
>>> +config RISCV_USE_QUEUED_SPINLOCKS
>> +       bool "Using queued spinlock instead of ticket-lock"
> Maybe we can just make ARCH_USE_QUEUED_SPINLOCKS
> user visible and give users the choice between the two generic
> implementations across all architectures that support the qspinlock
> variant.
>
> In arch/riscv, you'd then just have a
>
>          select ARCH_HAVE_QUEUED_SPINLOCKS
>
> diff --git a/arch/riscv/include/asm/spinlock.h
> b/arch/riscv/include/asm/spinlock.h
>> new file mode 100644
>> index 000000000000..fd3fd09cff52
>> --- /dev/null
>> +++ b/arch/riscv/include/asm/spinlock.h
>> @@ -0,0 +1,12 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef __ASM_SPINLOCK_H
>> +#define __ASM_SPINLOCK_H
>> +
>> +#ifdef CONFIG_ARCH_USE_QUEUED_SPINLOCKS
>> +#include <asm/qspinlock.h>
>> +#include <asm/qrwlock.h>
>> +#else
>> +#include <asm-generic/spinlock.h>
>> +#endif
>> +
> Along the same lines:
>
> I think I'd prefer the header changes to be done in the asm-generic
> version of this file, so this can be shared across all architectures
> that want to give the choice between ticket and queued spinlock.

I concur. Qspinlock is only needed if we want to support systems with a 
large number of CPUs. For systems with a small number of CPUs. It 
doesn't matter if qspinlock or the ticket lock is being used.

Cheers,
Longman