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

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Date: Wed, 6 Mar 2024 18:49:38 -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 18:43, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I dunno.

Oh, and just looking at that patch, I still think the code is confused.

On the reading side, we have:

    pipe_count = smp_load_acquire(&p->rtort_pipe_count);
    if (pipe_count > RCU_TORTURE_PIPE_LEN) {
        /* Should not happen, but... */

where that comment clearly says that the pipe_count we read (whether
with READ_ONCE() or with my smp_load_acquire() suggestion) should
never be larger than RCU_TORTURE_PIPE_LEN.

But the writing side very clearly did:

    i = rp->rtort_pipe_count;
    if (i > RCU_TORTURE_PIPE_LEN)
        i = RCU_TORTURE_PIPE_LEN;
    ...
    smp_store_release(&rp->rtort_pipe_count, ++i);

(again, syntactically it could have been "i + 1" instead of my "++i" -
same value), so clearly the writing side *can* write a value that is >
RCU_TORTURE_PIPE_LEN.

So while the whole READ/WRITE_ONCE vs smp_load_acquire/store_release
is one thing that might be worth looking at, I think there are other
very confusing aspects here.

           Linus