[PATCH 5/9] eventpoll: add ep_poll_queue() loop

[Jens Axboe <axboe@kernel.dk>]

If a wait_queue_entry is passed in to epoll_wait(), then utilize this
new helper for reaping events and/or adding to the epoll waitqueue
rather than calling the potentially sleeping ep_poll(). It works like
ep_poll(), except it doesn't block - it either returns the events that
are already available, or it adds the specified entry to the struct
eventpoll waitqueue to get a callback when events are triggered. It
returns -EIOCBQUEUED for that case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/eventpoll.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ecaa5591f4be..a8be0c7110e4 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2032,6 +2032,39 @@ static int ep_try_send_events(struct eventpoll *ep,
 	return res;
 }
 
+static int ep_poll_queue(struct eventpoll *ep,
+			 struct epoll_event __user *events, int maxevents,
+			 struct wait_queue_entry *wait)
+{
+	int res, eavail;
+
+	/* See ep_poll() for commentary */
+	eavail = ep_events_available(ep);
+	while (1) {
+		if (eavail) {
+			res = ep_try_send_events(ep, events, maxevents);
+			if (res)
+				return res;
+		}
+
+		eavail = ep_busy_loop(ep, true);
+		if (eavail)
+			continue;
+
+		if (!list_empty_careful(&wait->entry))
+			return -EIOCBQUEUED;
+
+		write_lock_irq(&ep->lock);
+		eavail = ep_events_available(ep);
+		if (!eavail)
+			__add_wait_queue_exclusive(&ep->wq, wait);
+		write_unlock_irq(&ep->lock);
+
+		if (!eavail)
+			return -EIOCBQUEUED;
+	}
+}
+
 /**
  * ep_poll - Retrieves ready events, and delivers them to the caller-supplied
  *           event buffer.
@@ -2497,7 +2530,9 @@ int epoll_wait(struct file *file, struct epoll_event __user *events,
 	ep = file->private_data;
 
 	/* Time to fish for events ... */
-	return ep_poll(ep, events, maxevents, to);
+	if (!wait)
+		return ep_poll(ep, events, maxevents, to);
+	return ep_poll_queue(ep, events, maxevents, wait);
 }
 
 /*
-- 
2.47.2