Re: [PATCH] wait_on_bit: add an acquire memory barrier

[Joel Fernandes]

On Mon, Aug 22, 2022 at 1:39 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 22, 2022 at 10:08 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > So why don't we just create a "test_bit_acquire()" and be done with
> > it? We literally created clear_bit_unlock() for the opposite reason,
> > and your comments about the new barrier hack even point to it.
>
> Here's a patch that is
>
>  (a) almost entirely untested (I checked that one single case builds
> and seems to generate the expected code)
>
>  (b) needs some more loving
>
> but seems to superficially work.
>
> At a minimum this needs to be split into two (so the bitop and the
> wait_on_bit parts split up), and that whole placement of
> <asm/barrier.h> and generic_bit_test_acquire() need at least some
> thinking about, but on the whole it seems reasonable.
>
> For example, it would make more sense to have this in
> <asm-generic/bitops/lock.h>, but not all architectures include that,
> and some do their own version. I didn't want to mess with
> architecture-specific headers, so this illogically just uses
> generic-non-atomic.h.
>
> Maybe just put it in <linux/bitops.h> directly?
>
> So I'm not at all claiming that this is a great patch. It definitely
> needs more work, and a lot more testing.
>
> But I think this is at least the right _direction_ to take here.
>
> And yes, I think it also would have been better if
> "clear_bit_unlock()" would have been called "clear_bit_release()", and
> we'd have more consistent naming with our ordered atomics. But it's
> probably not worth changing.

Also, as a suggestion to Mikulas or whoever works on this - update the
ORDERING section of Documentation/atomic_bitops.txt too?

Thanks,

 - Joel