linux-kernel - Re: [PATCH V2 1/5] asm-generic: ticket-lock: New generic ticket-based spinlock

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Date:   Sat, 19 Mar 2022 12:52:09 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>, Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-csky@...r.kernel.org,
        Openrisc <openrisc@...ts.librecores.org>,
        Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH V2 1/5] asm-generic: ticket-lock: New generic ticket-based spinlock

On Sat, Mar 19, 2022 at 4:54 AM <guoren@...nel.org> wrote:
>  /*
> - * You need to implement asm/spinlock.h for SMP support. The generic
> - * version does not handle SMP.
> + * Using ticket-spinlock.h as generic for SMP support.
>   */
>  #ifdef CONFIG_SMP
> -#error need an architecture specific asm/spinlock.h
> +#include <asm-generic/ticket-lock.h>
> +#ifdef CONFIG_QUEUED_RWLOCKS
> +#include <asm-generic/qrwlock.h>
> +#else
> +#error Please select ARCH_USE_QUEUED_RWLOCKS in architecture Kconfig
> +#endif
>  #endif

There is no need for the !CONFIG_SMP case, as asm/spinlock.h only ever
gets included for SMP builds in the first place. This was already a mistake
in the existing code, but your change would be the time to fix it.

I would also drop the !CONFIG_QUEUED_RWLOCKS case, just include
it unconditionally. If any architecture wants the ticket spinlock in
combination with a custom rwlock, they can simply include the
asm-generic/ticket-lock.h from their asm/spinlock.h, but more
likely any architecture that can use the ticket spinlock will also
want the qrwlock anyway.

     Arnd