linux-kernel - Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Date:   Wed, 31 Mar 2021 09:23:27 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Stafford Horne <shorne@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, Guo Ren <guoren@...nel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Anup Patel <anup@...infault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Wed, Mar 31, 2021 at 12:35 AM Stafford Horne <shorne@...il.com> wrote:
>
> I just want to chime in here, there may be a better spot in the thread to
> mention this but, for OpenRISC I did implement some generic 8/16-bit xchg code
> which I have on my todo list somwhere to replace the other generic
> implementations like that in mips.
>
>   arch/openrisc/include/asm/cmpxchg.h
>
> The idea would be that architectures just implement these methods:
>
>   long cmpxchg_u32(*ptr,old,new)
>   long xchg_u32(*ptr,val)
>
> Then the rest of the generic header would implement cmpxchg.

I like the idea of generalizing it a little further. I'd suggest staying a
little closer to the existing naming here though, as we already have
cmpxchg() for the type-agnostic version, and cmpxchg64() for the
fixed-length 64-bit version.

I think a nice interface between architecture-specific and architecture
independent code would be to have architectures provide
arch_cmpxchg32()/arch_xchg32() as the most basic version, as well
as arch_cmpxchg8()/arch_cmpxchg16()/arch_xchg8()/arch_xchg16()
if they have instructions for those.

The common code can then build cmpxchg16()/xchg16() on top of
either the 16-bit or the 32-bit primitives, and build the cmpxchg()/xchg()
wrapper around those (or alternatively we can decide to have them
only deal with fixed-32-bit and long/pointer sized atomics).

      Arnd