pipe: don't use 'pipe_wait() for basic pipe IO

pipe_wait() may be simple, but since it relies on the pipe lock, it
means that we have to do the wakeup while holding the lock.  That's
unfortunate, because the very first thing the waked entity will want to
do is to get the pipe lock for itself.

So get rid of the pipe_wait() usage by simply releasing the pipe lock,
doing the wakeup (if required) and then using wait_event_interruptible()
to wait on the right condition instead.

wait_event_interruptible() handles races on its own by comparing the
wakeup condition before and after adding itself to the wait queue, so
you can use an optimistic unlocked condition for it.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/pipe.c

@@ -270,6 +270,16 @@ static bool pipe_buf_can_merge(struct pipe_buffer *buf)
 	return buf->ops == &anon_pipe_buf_ops;
 }
 
+/* Done while waiting without holding the pipe lock - thus the READ_ONCE() */
+static inline bool pipe_readable(const struct pipe_inode_info *pipe)
+{
+	unsigned int head = READ_ONCE(pipe->head);
+	unsigned int tail = READ_ONCE(pipe->tail);
+	unsigned int writers = READ_ONCE(pipe->writers);
+
+	return !pipe_empty(head, tail) || !writers;
+}
+
 static ssize_t
 pipe_read(struct kiocb *iocb, struct iov_iter *to)
 {
@@ -359,11 +369,13 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				ret = -ERESTARTSYS;
 			break;
 		}
+		__pipe_unlock(pipe);
 		if (was_full) {
 			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
 			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 		}
-		pipe_wait(pipe);
+		wait_event_interruptible(pipe->wait, pipe_readable(pipe));
+		__pipe_lock(pipe);
 		was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
 	}
 	__pipe_unlock(pipe);
@@ -382,6 +394,17 @@ static inline int is_packetized(struct file *file)
 	return (file->f_flags & O_DIRECT) != 0;
 }
 
+/* Done while waiting without holding the pipe lock - thus the READ_ONCE() */
+static inline bool pipe_writable(const struct pipe_inode_info *pipe)
+{
+	unsigned int head = READ_ONCE(pipe->head);
+	unsigned int tail = READ_ONCE(pipe->tail);
+	unsigned int max_usage = READ_ONCE(pipe->max_usage);
+
+	return !pipe_full(head, tail, max_usage) ||
+		!READ_ONCE(pipe->readers);
+}
+
 static ssize_t
 pipe_write(struct kiocb *iocb, struct iov_iter *from)
 {
@@ -529,12 +552,13 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		 * after waiting we need to re-check whether the pipe
 		 * become empty while we dropped the lock.
 		 */
+		__pipe_unlock(pipe);
 		if (was_empty) {
 			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLRDNORM);
 			kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 		}
-		pipe_wait(pipe);
-
+		wait_event_interruptible(pipe->wait, pipe_writable(pipe));
+		__pipe_lock(pipe);
 		was_empty = pipe_empty(head, pipe->tail);
 	}
 out: