Re: [PATCH v3 1/2] wait_bit: do read barrier after testing a bit

[Linus Torvalds]

On Sun, Jul 31, 2022 at 1:41 PM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
> -       if (!test_bit(bit, word))
> +       if (!test_bit(bit, word)) {
> +               smp_rmb();

Logically, I don't think that makes sense.

Maybe you're checking the buffer being up-to-date before you *write* to it?

So smp_rmb() seems entirely wrong.

I think it should consistently aim for just doing

        unsigned long state = smp_read_acquire(word);
        if (!(state & (1 << bit)))
                return 0;

or whatever.

We should strive to *not* add new uses of the legacy memory barriers.
They are garbage from last century when people didn't know better.

Then people learnt to use acquire and release, and things improved.
Let's live in that improved world.

                 Linus