On Wed, 6 Mar 2024 at 11:45, Steven Rostedt <rost...@goodmis.org> wrote:
>
> Here's the back story. I received the following patch:
>
>
> https://lore.kernel.org/all/tencent_ba1473492bc618b473864561ea3ab1418...@qq.com/
>
> I didn't like it. My reply was:
>
> > - rbwork->wait_index++;
> > + WRITE_ONCE(rbwork->wait_index, READ_ONCE(rbwork->wait_index)
> + 1);
>
> I mean the above is really ugly. If this is the new thing to do, we
> need
> better macros.
>
> If anything, just convert it to an atomic_t.
The right thing is definitely to convert it to an atomic_t.
The memory barriers can probably also be turned into atomic ordering,
although we don't always have all the variates.
But for example, that
/* Make sure to see the new wait index */
smp_rmb();
if (wait_index != work->wait_index)
break;
looks odd, and should probably do an "atomic_read_acquire()" instead
of a rmb and a (non-atomic and non-READ_ONCE thing).
The first READ_ONCE() should probably also be that atomic_read_acquire() op.
On the writing side, my gut feel is that the
rbwork->wait_index++;
/* make sure the waiters see the new index */
smp_wmb();
should be an "atomic_inc_release(&rbwork->wait_index);" but we don't
actually have that operation. We only have the "release" versions for
things that return a value.
So it would probably need to be either
atomic_inc(&rbwork->wait_index);
/* make sure the waiters see the new index */
smp_wmb();
or
atomic_inc_return_release(&rbwork->wait_index);
or we'd need to add the "basic atomics with ordering semantics" (which
we aren't going to do unless we end up with a lot more people who want
them).
I dunno. I didn't look all *that* closely at the code. The above might
be garbage too. Somebody who actually knows the code should think
about what ordering they actually were looking for.
(And I note that 'wait_index' is of type 'long' in 'struct
rb_irq_work', so I guess it should be "atomic_long_t" instead - just
shows how little attention I paid on the first read-through, which
should make everybody go "I need to double-check Linus here")
Linus
