Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll_callback() contention

[Linus Torvalds]

On Mon, Dec 3, 2018 at 3:03 AM Roman Penyaev <rpenyaev@xxxxxxx> wrote:
>
> Also I'm not quite sure where to put very special lockless variant
> of adding element to the list (list_add_tail_lockless() in this
> patch).  Seems keeping it locally is safer.

That function is scary, and can be mis-used so easily that I
definitely don't want to see it anywhere else.

Afaik, it's *really* important that only "add_tail" operations can be
done in parallel.

This also ends up making the memory ordering of "xchg()" very very
important. Yes, we've documented it as being an ordering op, but I'm
not sure we've relied on it this directly before.

I also note that now we do more/different locking in the waitqueue
handling, because the code now takes both that rwlock _and_ the
waitqueue spinlock for wakeup. That also makes me worried that the
"waitqueue_active()" games are no no longer reliable. I think they're
fine (looks like they are only done under the write-lock, so it's
effectively the same serialization anyway), but the upshoot of all of
this is that I *really* want others to look at this patch too. A lot
of small subtle things here.

                Linus