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

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Date:   Mon, 29 Mar 2021 09:50:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     guoren@...nel.org
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-csky@...r.kernel.org, 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>,
        Arnd Bergmann <arnd@...db.de>, Anup Patel <anup@...infault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Sat, Mar 27, 2021 at 06:06:38PM +0000, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
> 
> Some architectures don't have sub-word swap atomic instruction,
> they only have the full word's one.
> 
> The sub-word swap only improve the performance when:
> NR_CPUS < 16K
>  *  0- 7: locked byte
>  *     8: pending
>  *  9-15: not used
>  * 16-17: tail index
>  * 18-31: tail cpu (+1)
> 
> The 9-15 bits are wasted to use xchg16 in xchg_tail.
> 
> Please let architecture select xchg16/xchg32 to implement
> xchg_tail.

So I really don't like this, this pushes complexity into the generic
code for something that's really not needed.

Lots of RISC already implement sub-word atomics using word ll/sc.
Obviously they're not sharing code like they should be :/ See for
example arch/mips/kernel/cmpxchg.c.

Also, I really do think doing ticket locks first is a far more sensible
step.