Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 27, 2020 at 09:32:11PM +0000, David Woodhouse wrote:
> 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 :-(
> 
> I think that goes all the way back to here:
> 
> https://lkml.org/lkml/2007/5/4/530
> 
> It was rounded up in commit d47de16c72and subsequently "cleaned up"
> into an inline in wait.h, but I don't think there was ever a reason for
> it to be added to the head of the list instead of the tail.

Maybe, I'm not sure I can tell in a hurry. I've opted to undo the above
'cleanups'

> So I think we can reasonably make __add_wait_queue_exclusive() do
> precisely the same thing as add_wait_queue_exclusive() does (modulo
> locking).

Aye, see below.

> And then potentially rename them both to something that isn't quite
> such a lie. And give me the one I want that *does* actually exclude
> other waiters :)

I don't think we want to do that; people are very much used to the
current semantics.

I also very much want to do:
s/__add_wait_queue_entry_tail/__add_wait_queue_tail/ on top of all this.

Anyway, I'll agree to your patch. How do we route this? Shall I take the
waitqueue thing and stick it in a topic branch for Paolo so he can then
merge that and the kvm bits on top into the KVM tree?

---
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 4df61129566d..a2a7e1e339f6 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1895,10 +1895,12 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 */
 		eavail = ep_events_available(ep);
 		if (!eavail) {
-			if (signal_pending(current))
+			if (signal_pending(current)) {
 				res = -EINTR;
-			else
-				__add_wait_queue_exclusive(&ep->wq, &wait);
+			} else {
+				wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
+				__add_wait_queue(wq_head, wq_entry);
+			}
 		}
 		write_unlock_irq(&ep->lock);
 
diff --git a/fs/orangefs/orangefs-bufmap.c b/fs/orangefs/orangefs-bufmap.c
index 538e839590ef..8cac3589f365 100644
--- a/fs/orangefs/orangefs-bufmap.c
+++ b/fs/orangefs/orangefs-bufmap.c
@@ -86,7 +86,7 @@ static int wait_for_free(struct slot_map *m)
 	do {
 		long n = left, t;
 		if (likely(list_empty(&wait.entry)))
-			__add_wait_queue_entry_tail_exclusive(&m->q, &wait);
+			__add_wait_queue_exclusive(&m->q, &wait);
 		set_current_state(TASK_INTERRUPTIBLE);
 
 		if (m->c > 0)
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 27fb99cfeb02..4b8c4ece13f7 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -171,23 +171,13 @@ static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait
 	list_add(&wq_entry->entry, &wq_head->head);
 }
 
-/*
- * Used for wake-one threads:
- */
-static inline void
-__add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
-{
-	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
-	__add_wait_queue(wq_head, wq_entry);
-}
-
 static inline void __add_wait_queue_entry_tail(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
 	list_add_tail(&wq_entry->entry, &wq_head->head);
 }
 
 static inline void
-__add_wait_queue_entry_tail_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
+__add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 {
 	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
 	__add_wait_queue_entry_tail(wq_head, wq_entry);