Re: [PATCH v3 2/2] make buffer_locked provide an acquire semantics

[Linus Torvalds]

On Sun, Jul 31, 2022 at 1:43 PM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
> +
> +static __always_inline int buffer_locked(const struct buffer_head *bh)
> +{
> +       unsigned long state = smp_load_acquire(&bh->b_state);
> +       return test_bit(BH_Lock, &state);

This should not use 'test_bit()'. I suspect that generates horrendous
code, because it's a volatile access, so now you'll load it into a
register, and I suspect it will generate s pointless spill just to do
a volatile load.

I didn't check.

So once you've loaded b_state, just test the bit directly with

    return (state & (1u << BH_Lock)) != 0;

or whatever.

              Linus