pipe: simplify signal handling in pipe_read() and add comments

			ret = -EAGAIN;

			break;

		}

		if (signal_pending(current)) {

			if (!ret)

				ret = -ERESTARTSYS;

			break;

		}

		__pipe_unlock(pipe);

		if (was_full) {

		/*

		 * We only get here if we didn't actually read anything.

		 *

		 * However, we could have seen (and removed) a zero-sized

		 * pipe buffer, and might have made space in the buffers

		 * that way.

		 *

		 * You can't make zero-sized pipe buffers by doing an empty

		 * write (not even in packet mode), but they can happen if

		 * the writer gets an EFAULT when trying to fill a buffer

		 * that already got allocated and inserted in the buffer

		 * array.

		 *

		 * So we still need to wake up any pending writers in the

		 * _very_ unlikely case that the pipe was full, but we got

		 * no data.

		 */

		if (unlikely(was_full)) {

			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);

			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);

		}

		wait_event_interruptible(pipe->wait, pipe_readable(pipe));

		/*

		 * But because we didn't read anything, at this point we can

		 * just return directly with -ERESTARTSYS if we're interrupted,

		 * since we've done any required wakeups and there's no need

		 * to mark anything accessed. And we've dropped the lock.

		 */

		if (wait_event_interruptible(pipe->wait, pipe_readable(pipe)) < 0)

			return -ERESTARTSYS;

		__pipe_lock(pipe);

		was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);

	}