linux-kernel - Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture

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Date: Wed, 6 Mar 2024 18:43:42 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: paulmck@...nel.org
Cc: Steven Rostedt <rostedt@...dmis.org>, linke li <lilinke99@...com>, joel@...lfernandes.org, 
	boqun.feng@...il.com, dave@...olabs.net, frederic@...nel.org, 
	jiangshanlai@...il.com, josh@...htriplett.org, linux-kernel@...r.kernel.org, 
	mathieu.desnoyers@...icios.com, qiang.zhang1211@...il.com, 
	quic_neeraju@...cinc.com, rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture_writer()
 data race and concurrency bug

On Wed, 6 Mar 2024 at 18:29, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> TL;DR:  Those ->rtort_pipe_count increments cannot run concurrently
> with each other or any other update of that field, so that update-side
> READ_ONCE() call is unnecessary and the update-side plain C-language
> read is OK.  The WRITE_ONCE() calls are there for the benefit of the
> lockless read-side accesses to rtort_pipe_count.

Ahh. Ok. That makes a bit more sense.

So if that's the case, then the "updating side" should never use
READ_ONCE, because there's nothing else to protect against.

Honestly, this all makes me think that we'd be *much* better off
showing the real "handoff" with smp_store_release() and
smp_load_acquire().

IOW, something like this (TOTALLY UNTESTED!) patch, perhaps?

And please note that this patch is not only untested, it really is a
very handwavy patch.

I'm sending it as a patch just because it's a more precise way of
saying "I think the writers and readers could use the store-release ->
load-acquire not just to avoid any worries about accessing things
once, but also as a way to show the directional 'flow' of the data".

I dunno.

           Linus

View attachment "patch.diff" of type "text/x-patch" (1690 bytes)