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

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Date: Fri, 3 May 2024 00:24:47 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	elver@...gle.com, akpm@...ux-foundation.org, tglx@...utronix.de,
	peterz@...radead.org, dianders@...omium.org, pmladek@...e.com,
	arnd@...db.de, 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
Subject: Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

On Thu, May 02, 2024 at 04:12:44PM -0700, Paul E. McKenney wrote:

> > I'm probably missing your point, though - what mix of cmpxchg and
> > smp_store_release on 8bit values?
> 
> One of RCU's state machines uses smp_store_release() to start the
> state machine (only one task gets to do this) and cmpxchg() to update
> state beyond that point.  And the state is 8 bits so that it and other
> state fits into 32 bits to allow a single check for multiple conditions
> elsewhere.

Humm...  smp_store_release() of 8bit on old alpha is mb + fetch 64bit + replace
8 bits + store 64bit...