epoll: simplify signal handling

Check signals before locking ep->lock, and immediately return -EINTR if
there is any signal pending.

This saves a few loads, stores, and branches from the hot path and
simplifies the loop structure for follow up patches.

Link: https://lkml.kernel.org/r/20201106231635.3528496-3-soheil.kdev@gmail.comSigned-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
Cc: Guantao Liu <guantaol@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1733,7 +1733,7 @@ static inline struct timespec64 ep_set_mstimeout(long ms)
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		   int maxevents, long timeout)
 {
-	int res = 0, eavail, timed_out = 0;
+	int res, eavail, timed_out = 0;
 	u64 slack = 0;
 	wait_queue_entry_t wait;
 	ktime_t expires, *to = NULL;
@@ -1780,6 +1780,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 	ep_reset_busy_poll_napi_id(ep);
 
 	do {
+		if (signal_pending(current))
+			return -EINTR;
+
 		/*
 		 * Internally init_wait() uses autoremove_wake_function(),
 		 * thus wait entry is removed from the wait queue on each
@@ -1809,15 +1812,12 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * important.
 		 */
 		eavail = ep_events_available(ep);
-		if (!eavail) {
-			if (signal_pending(current))
-				res = -EINTR;
-			else
-				__add_wait_queue_exclusive(&ep->wq, &wait);
-		}
+		if (!eavail)
+			__add_wait_queue_exclusive(&ep->wq, &wait);
+
 		write_unlock_irq(&ep->lock);
 
-		if (!eavail && !res)
+		if (!eavail)
 			timed_out = !schedule_hrtimeout_range(to, slack,
 							      HRTIMER_MODE_ABS);
 
@@ -1853,14 +1853,14 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * finding more events available and fetching
 		 * repeatedly.
 		 */
-		res = -EINTR;
+		return -EINTR;
 	}
 	/*
 	 * Try to transfer events to user space. In case we get 0 events and
 	 * there's still timeout left over, we go trying again in search of
 	 * more luck.
 	 */
-	if (!res && eavail &&
+	if (eavail &&
 	    !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
 		goto fetch_events;