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

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Date: Wed, 6 Mar 2024 13:20:12 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: 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,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] rcutorture: Fix
 rcu_torture_pipe_update_one()/rcu_torture_writer() data race and
 concurrency bug

On Wed, 6 Mar 2024 10:09:48 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > Perhaps we need a way to annotate them, like we have with __rcu. "__shared"?
> > 
> > Then all accesses to that variable must be wrapped with a READ_ONCE() or
> > WRITE_ONCE()? I mean, if this can cause legitimate bugs, we should probably
> > address it like we do with locking and RCU.  
> 
> If we want that, just mark the field "volatile", as in "jiffies".

I already know Linus's view on "volatile" variables ;-)

> 
> And one of the strengths of READ_ONCE() and WRITE_ONCE() is that they
> allow non-volatile access where it is safe.  For example, if you hold the
> lock protecting all stores to that variable, you still need WRITE_ONCE()
> but not READ_ONCE().  In initialization and cleanup code, you don't
> need either.

I guess the current static analyzers just look to see where READ_ONCE() or
WRITE_ONCE() is used and checks to see if other places have them properly
used. I'm guessing that's where the OP patch came from.

Sounds like we just need a ADD_ONCE() or INC_ONCE() then. Because I am not
taking a

	WRITE_ONCE(a, READ_ONCE(a) + 1);

patch that replaces a simple "a++".

-- Steve