pipe: fix empty pipe check in pipe_write()

LTP pipeio_1 test is hanging with v5.5-rc2-385-gb8e382a1,
with read side observing empty pipe and sleeping and write
side running out of space and then sleeping as well. In this
scenario there are 5 writers and 1 reader.

Problem is that after pipe_write() reacquires pipe lock, it
re-checks for empty pipe with potentially stale 'head' and
doesn't wake up read side anymore. pipe->tail can advance
beyond 'head', because there are multiple writers.

Use pipe->head for empty pipe check after reacquiring lock
to observe current state.

Testing: With patch, LTP pipeio_1 ran successfully in loop for 1 hour.
         Without patch it hanged within a minute.

Fixes: 1b6b26ae ("pipe: fix and clarify pipe write wakeup logic")
Reported-by: Rachel Sibley <rasibley@redhat.com>
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/pipe.c

@@ -581,7 +581,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		}
 		wait_event_interruptible(pipe->wait, pipe_writable(pipe));
 		__pipe_lock(pipe);
-		was_empty = pipe_empty(head, pipe->tail);
+		was_empty = pipe_empty(pipe->head, pipe->tail);
 	}
 out:
 	__pipe_unlock(pipe);