linux-kernel - Re: [PATCH 4/7] riscv: Implement xchg8/16() using Zabha

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Message-ID: <ZlYbupL5XgzgA0MX@andrea>
Date: Tue, 28 May 2024 20:00:26 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	Arnd Bergmann <arnd@...db.de>, Leonardo Bras <leobras@...hat.com>,
	Guo Ren <guoren@...nel.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 4/7] riscv: Implement xchg8/16() using Zabha

> -#define __arch_xchg_masked(prepend, append, r, p, n)			\
> +#define __arch_xchg_masked(swap_sfx, prepend, append, r, p, n)		\

This actually indicates a problem in the current (aka, no Zabha)
implementation: without your series, xchg16() gets mapped to

  lr.w     a2,(a3)
  and      a1,a2,a5
  or       a1,a1,a4
  sc.w     a1,a1,(a3)
  bnez     a1,43c <.L0^B1>

which is clearly wrong... (other "fully-ordered LR/SC sequences"
instead follow the mapping

  lr.w     a2,(a3)
  and      a1,a2,a5
  or       a1,a1,a4
  sc.w.rl  a1,a1,(a3)
  bnez     a1,43c <.L0^B1>
  fence    rw,rw  )

A similar consideration for xchg8().

  Andrea