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

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Date: Thu, 2 May 2024 16:32:35 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: paulmck@...nel.org
Cc: Al Viro <viro@...iv.linux.org.uk>, 
	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, 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, 2 May 2024 at 16:12, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> 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.

Note that since alpha lacks the release-acquire model, it's always
going to be a full memory barrier before the store.

And then the store turns into a load-mask-store for older alphas.

So it's going to be a complete mess from a performance standpoint regardless.

Happily, I doubt anybody really cares.

I've occasionally wondered if we have situations where the
"smp_store_release()" only cares about previous *writes* being ordered
(ie a "smp_wmb()+WRITE_ONCE" would be sufficient).

It makes no difference on x86 (all stores are relases), power64 (wmb
and store_release are both LWSYNC) or arm64 (str is documentated to be
cheaper than DMB).

On alpha, smp_wmb()+WRITE_ONCE() is cheaper than smp_store_release(),
but nobody sane cares.

But *if* we have a situation where the "smp_store_release()" might be
just a "previous writes need to be visible" rather than ordering
previous reads too, we could maybe introduce that kind of op. I
_think_ the RCU writes tend to be of that kind?

                    Linus