On Sun, 31 Jul 2022, Linus Torvalds wrote:
> 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?
None of the CPUs have speculative writes - so the write can't be moved
before the "test_bit" function. So, we are only concerned about reads.
> 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
OK - I'm sending new patches that introduce the function test_bit_acquire.
Mikulas
