pipe: Conditionalise wakeup in pipe_read()

Only do a wakeup in pipe_read() if we made space in a completely full
buffer.  The producer shouldn't be waiting on pipe->wait otherwise.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/pipe.c

@@ -328,11 +328,13 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				spin_lock_irq(&pipe->wait.lock);
 				tail++;
 				pipe->tail = tail;
-				do_wakeup = 0;
-				wake_up_interruptible_sync_poll_locked(
-					&pipe->wait, EPOLLOUT | EPOLLWRNORM);
+				do_wakeup = 1;
+				if (head - (tail - 1) == pipe->max_usage)
+					wake_up_interruptible_sync_poll_locked(
+						&pipe->wait, EPOLLOUT | EPOLLWRNORM);
 				spin_unlock_irq(&pipe->wait.lock);
-				kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
+				if (head - (tail - 1) == pipe->max_usage)
+					kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 			}
 			total_len -= chars;
 			if (!total_len)
@@ -361,11 +363,6 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				ret = -ERESTARTSYS;
 			break;
 		}
-		if (do_wakeup) {
-			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
- 			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-			do_wakeup = 0;
-		}
 		pipe_wait(pipe);
 	}
 	__pipe_unlock(pipe);