On Tue, 2020-10-27 at 21:30 +0100, Peter Zijlstra wrote: > On Tue, Oct 27, 2020 at 07:27:59PM +0000, David Woodhouse wrote: > > > > While looking at this I found that weird __add_wait_queue_exclusive() > > > which is used by fs/eventpoll.c and does something similar, except it > > > doesn't keep the FIFO order. > > > > It does, doesn't it? Except those so-called "exclusive" entries end up > > in FIFO order amongst themselves at the *tail* of the queue, to be > > woken up only after all the other entries before them *haven't* been > > excluded. > > __add_wait_queue_exclusive() uses __add_wait_queue() which does > list_add(). It does _not_ add at the tail like normal exclusive users, > and there is exactly _1_ user in tree that does this. > > I'm not exactly sure how this happened, but: > > add_wait_queue_exclusive() > > and > > __add_wait_queue_exclusive() > > are not related :-(
Oh, that is *particularly* special.
It sounds like the __add_wait_queue_exclusive() version is a half-baked
attempt at doing what I'm doing here, except....
> > > The Changelog doesn't state how important this property is to you.
> >
> > Because it isn't :)
> >
> > The ordering is:
> >
> > { PRIORITY }* { NON-EXCLUSIVE }* { EXCLUSIVE(sic) }*
> >
> > I care that PRIORITY comes before the others, because I want to
> > actually exclude the others. Especially the "non-exclusive" ones, which
> > the 'exclusive' ones don't actually exclude.
> >
> > I absolutely don't care about ordering *within* the set of PRIORITY
> > entries, since as I said I expect there to be only one.
>
> Then you could arguably do something like:
>
> spin_lock_irqsave(&wq_head->lock, flags);
> __add_wait_queue_exclusive(wq_head, wq_entry);
> spin_unlock_irqrestore(&wq_head->lock, flags);
>
> and leave it at that.
.. the problem with that is that other waiters *can* end up on the
queue before it, if they are added later. I don't know if the existing
user (ep_poll) cares, but I do.
> But now I'm itching to fix that horrible naming... tomorrow perhaps.
:)
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