linux-kernel - Re: [OpenRISC] [PATCH v6 1/9] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Date:   Thu, 8 Apr 2021 05:12:58 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, guoren@...nel.org,
        linux-arch@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Arnd Bergmann <arnd@...db.de>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, linux-csky@...r.kernel.org,
        openrisc@...ts.librecores.org, Anup Patel <anup@...infault.org>,
        sparclinux@...r.kernel.org, Waiman Long <longman@...hat.com>,
        linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [OpenRISC] [PATCH v6 1/9] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Wed, Apr 07, 2021 at 11:47:49AM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 08:52:08AM +0900, Stafford Horne wrote:
> > Why doesn't RISC-V add the xchg16 emulation code similar to OpenRISC?  For
> > OpenRISC we added xchg16 and xchg8 emulation code to enable qspinlocks.  So
> > one thought is with CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32=y, can we remove our
> > xchg16/xchg8 emulation code?
> 
> CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32 is guaranteed crap.
>
> All the architectures that have wanted it are RISC style LL/SC archs,
> and for them a cmpxchg loop is a daft thing to do, since it reduces the
> chance of it behaving sanely.
> 
> Why would we provide something that's known to be suboptimal? If an
> architecture chooses to not care about determinism and or fwd progress,
> then that's their choice. But not one, I feel, we should encourage.

Thanks, this is the response I was hoping my comment would provoke.

So not enabling CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32 for architectures
unless they really want it should be the way.

-Stafford