[PATCH] pipe_read: don't wake up the writer if the pipe is still full

[Oleg Nesterov <oleg@redhat.com>]

wake_up(pipe->wr_wait) makes no sense if pipe_full() is still true after
the reading, the writer sleeping in wait_event(wr_wait, pipe_writable())
will check the pipe_writable() == !pipe_full() condition and sleep again.

Only wake the writer if we actually released a pipe buf, and the pipe was
full before we did so.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/pipe.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 12b22c2723b7..82fede0f2111 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -253,7 +253,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 	size_t total_len = iov_iter_count(to);
 	struct file *filp = iocb->ki_filp;
 	struct pipe_inode_info *pipe = filp->private_data;
-	bool was_full, wake_next_reader = false;
+	bool wake_writer = false, wake_next_reader = false;
 	ssize_t ret;
 
 	/* Null read succeeds. */
@@ -264,14 +264,13 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 	mutex_lock(&pipe->mutex);
 
 	/*
-	 * We only wake up writers if the pipe was full when we started
-	 * reading in order to avoid unnecessary wakeups.
+	 * We only wake up writers if the pipe was full when we started reading
+	 * and it is no longer full after reading to avoid unnecessary wakeups.
 	 *
 	 * But when we do wake up writers, we do so using a sync wakeup
 	 * (WF_SYNC), because we want them to get going and generate more
 	 * data for us.
 	 */
-	was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
 	for (;;) {
 		/* Read ->head with a barrier vs post_one_notification() */
 		unsigned int head = smp_load_acquire(&pipe->head);
@@ -340,8 +339,10 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				buf->len = 0;
 			}
 
-			if (!buf->len)
+			if (!buf->len) {
+				wake_writer |= pipe_full(head, tail, pipe->max_usage);
 				tail = pipe_update_tail(pipe, buf, tail);
+			}
 			total_len -= chars;
 			if (!total_len)
 				break;	/* common path: read succeeded */
@@ -377,7 +378,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		 * _very_ unlikely case that the pipe was full, but we got
 		 * no data.
 		 */
-		if (unlikely(was_full))
+		if (unlikely(wake_writer))
 			wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
 		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 
@@ -390,15 +391,15 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		if (wait_event_interruptible_exclusive(pipe->rd_wait, pipe_readable(pipe)) < 0)
 			return -ERESTARTSYS;
 
-		mutex_lock(&pipe->mutex);
-		was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
+		wake_writer = false;
 		wake_next_reader = true;
+		mutex_lock(&pipe->mutex);
 	}
 	if (pipe_empty(pipe->head, pipe->tail))
 		wake_next_reader = false;
 	mutex_unlock(&pipe->mutex);
 
-	if (was_full)
+	if (wake_writer)
 		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
 	if (wake_next_reader)
 		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
-- 
2.25.1.362.g51ebf55