fs/epoll: simplify ep_send_events_proc() ready-list loop

The current logic is a bit convoluted.  Lets simplify this with a
standard list_for_each_entry_safe() loop instead and just break out
after maxevents is reached.

While at it, remove an unnecessary indentation level in the loop when
there are in fact ready events.

Link: http://lkml.kernel.org/r/20181108051006.18751-3-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1624,21 +1624,22 @@ static __poll_t ep_send_events_proc(struct eventpoll *ep, struct list_head *head
 {
 	struct ep_send_events_data *esed = priv;
 	__poll_t revents;
-	struct epitem *epi;
-	struct epoll_event __user *uevent;
+	struct epitem *epi, *tmp;
+	struct epoll_event __user *uevent = esed->events;
 	struct wakeup_source *ws;
 	poll_table pt;
 
 	init_poll_funcptr(&pt, NULL);
+	esed->res = 0;
 
 	/*
 	 * We can loop without lock because we are passed a task private list.
 	 * Items cannot vanish during the loop because ep_scan_ready_list() is
 	 * holding "mtx" during this call.
 	 */
-	for (esed->res = 0, uevent = esed->events;
-	     !list_empty(head) && esed->res < esed->maxevents;) {
-		epi = list_first_entry(head, struct epitem, rdllink);
+	list_for_each_entry_safe(epi, tmp, head, rdllink) {
+		if (esed->res >= esed->maxevents)
+			break;
 
 		/*
 		 * Activate ep->ws before deactivating epi->ws to prevent
@@ -1658,42 +1659,42 @@ static __poll_t ep_send_events_proc(struct eventpoll *ep, struct list_head *head
 
 		list_del_init(&epi->rdllink);
 
-		revents = ep_item_poll(epi, &pt, 1);
-
 		/*
 		 * If the event mask intersect the caller-requested one,
 		 * deliver the event to userspace. Again, ep_scan_ready_list()
-		 * is holding "mtx", so no operations coming from userspace
+		 * is holding ep->mtx, so no operations coming from userspace
 		 * can change the item.
 		 */
-		if (revents) {
-			if (__put_user(revents, &uevent->events) ||
-			    __put_user(epi->event.data, &uevent->data)) {
-				list_add(&epi->rdllink, head);
-				ep_pm_stay_awake(epi);
-				if (!esed->res)
-					esed->res = -EFAULT;
-				return 0;
-			}
-			esed->res++;
-			uevent++;
-			if (epi->event.events & EPOLLONESHOT)
-				epi->event.events &= EP_PRIVATE_BITS;
-			else if (!(epi->event.events & EPOLLET)) {
-				/*
-				 * If this file has been added with Level
-				 * Trigger mode, we need to insert back inside
-				 * the ready list, so that the next call to
-				 * epoll_wait() will check again the events
-				 * availability. At this point, no one can insert
-				 * into ep->rdllist besides us. The epoll_ctl()
-				 * callers are locked out by
-				 * ep_scan_ready_list() holding "mtx" and the
-				 * poll callback will queue them in ep->ovflist.
-				 */
-				list_add_tail(&epi->rdllink, &ep->rdllist);
-				ep_pm_stay_awake(epi);
-			}
+		revents = ep_item_poll(epi, &pt, 1);
+		if (!revents)
+			continue;
+
+		if (__put_user(revents, &uevent->events) ||
+		    __put_user(epi->event.data, &uevent->data)) {
+			list_add(&epi->rdllink, head);
+			ep_pm_stay_awake(epi);
+			if (!esed->res)
+				esed->res = -EFAULT;
+			return 0;
+		}
+		esed->res++;
+		uevent++;
+		if (epi->event.events & EPOLLONESHOT)
+			epi->event.events &= EP_PRIVATE_BITS;
+		else if (!(epi->event.events & EPOLLET)) {
+			/*
+			 * If this file has been added with Level
+			 * Trigger mode, we need to insert back inside
+			 * the ready list, so that the next call to
+			 * epoll_wait() will check again the events
+			 * availability. At this point, no one can insert
+			 * into ep->rdllist besides us. The epoll_ctl()
+			 * callers are locked out by
+			 * ep_scan_ready_list() holding "mtx" and the
+			 * poll callback will queue them in ep->ovflist.
+			 */
+			list_add_tail(&epi->rdllink, &ep->rdllist);
+			ep_pm_stay_awake(epi);
 		}
 	}