linux-kernel - Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

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Message-Id: <989112a7-c12a-40be-9c0d-e516880380ad@app.fastmail.com>
Date: Thu, 02 May 2024 23:50:21 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Paul E. McKenney" <paulmck@...nel.org>,
 "John Paul Adrian Glaubitz" <glaubitz@...sik.fu-berlin.de>
Cc: Linux-Arch <linux-arch@...r.kernel.org>, linux-kernel@...r.kernel.org,
 "Marco Elver" <elver@...gle.com>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Thomas Gleixner" <tglx@...utronix.de>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Doug Anderson" <dianders@...omium.org>, "Petr Mladek" <pmladek@...e.com>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>, kernel-team@...a.com,
 "Andi Shyti" <andi.shyti@...ux.intel.com>,
 "Palmer Dabbelt" <palmer@...osinc.com>,
 "Masami Hiramatsu" <mhiramat@...nel.org>, linux-sh@...r.kernel.org,
 "Alexander Viro" <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

On Thu, May 2, 2024, at 15:33, Paul E. McKenney wrote:
> On Thu, May 02, 2024 at 07:11:52AM +0200, John Paul Adrian Glaubitz wrote:
>> On Wed, 2024-05-01 at 22:06 -0700, Paul E. McKenney wrote:
>> > > Does cmpxchg_emu_u8() have any advantages over the native xchg_u8()?
>> > 
>> > That would be 8-bit xchg() rather than 8-byte cmpxchg(), correct?
>> 
>> Indeed. I realized this after sending my reply.
>
> So this one-byte-only series affects only Alpha systems lacking
> single-byte load/store instructions.  If I understand correctly, Alpha
> 21164A (EV56) and later *do* have single-byte load/store instructions,
> and thus are still just fine.  In fact, it looks like EV56 also has
> two-byte load/store instructions, and so would have been OK with
> the original one-/two-byte RFC series.

Correct, the only other architecture I'm aware of that is missing
16-bit load/store entirely is ARMv3.

> Arnd will not be shy about correcting me if I am wrong.  ;-)

I'll take this as a reminder to send out my EV4/EV5 removal
series. I've merged my patches with Al's bugfixes and rebased
all on top of 6.9-rc now. It's a bit late now, so I'll
send this tomorrow:

https://git.kernel.org/pub/scm/linux/kernel/garch/alpha/include/asm/cmpxchg.hit/arnd/asm-generic.git/log/?h=alpha-cleanup-6.9

     Arnd