Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture_writer() data race and concurrency bug

[Linus Torvalds]

On Wed, 6 Mar 2024 at 18:43, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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