Received-SPF: pass (google.com: domain of linux-kernel+bounces-94456-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Precedence: bulk
MIME-Version: 1.0
References: <f3624f39-bbb1-451d-8161-8518e4108d8e@joelfernandes.org>
 <tencent_9882B228F292088CDD68F10CF1C228742009@qq.com> <20240306103719.1d241b93@gandalf.local.home>
 <27665890-8314-4252-8622-1e019fee27e4@paulmck-laptop> <20240306130103.6da71ddf@gandalf.local.home>
 <CAHk-=wgG6Dmt1JTXDbrbXh_6s2yLjL=9pHo7uv0==LHFD+aBtg@mail.gmail.com> <20240306135504.2b3872ef@gandalf.local.home>
In-Reply-To: <20240306135504.2b3872ef@gandalf.local.home>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 6 Mar 2024 11:01:55 -0800
Message-ID: <CAHk-=wjbDgMKLgxbV+yK4LKZ+2Qj6zVL_sHeb+L9KDia980Q8Q@mail.gmail.com>
Subject: Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture_writer()
 data race and concurrency bug
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>, linke li <lilinke99@qq.com>, joel@joelfernandes.org, 
	boqun.feng@gmail.com, dave@stgolabs.net, frederic@kernel.org, 
	jiangshanlai@gmail.com, josh@joshtriplett.org, linux-kernel@vger.kernel.org, 
	mathieu.desnoyers@efficios.com, qiang.zhang1211@gmail.com, 
	quic_neeraju@quicinc.com, rcu@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Wed, 6 Mar 2024 at 10:53, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> Now, are you OK with an addition of ADD_ONCE() and/or INC_ONCE()? So that we
> don't have to look at:
>
>         WRITE_ONCE(a, READ_ONCE(a) + 1);
>
> ?

In a generic header file under include/linux/?

Absolutely not. The above is a completely broken operation. There is
no way in hell we should expose it as a general helper.

So there is no way we'd add that kind of sh*t-for-brains operation in
(for example) our <asm/rwonce.h> header file next to the normal
READ/WRITE_ONCE defines.

In some individual tracing C file where it has a comment above it how
it's braindamaged and unsafe and talking about why it's ok in that
particular context? Go wild.

But honestly, I do not see when a ADD_ONCE() would ever be a valid
thing to do, and *if* it's a valid thing to do, why you'd do it with
READ_ONCE and WRITE_ONCE.

If you don't care about races, just do a simple "++" and be done with
it. The end result is random.

Adding a "ADD_ONCE()" macro doesn't make it one whit less random. It
just makes a broken concept even uglier.

So honestly, I think the ADD_ONCE macro not only needs to be in some
tracing-specific C file, the comment needs to be pretty damn big too.
Because as a random number generator, it's not even a very good one.
So you need to explain *why* you want a particularly bad random number
generator in the first place.

                  Linus