Unverified Commit aaec5a95 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Christian Brauner
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pipe_read: don't wake up the writer if the pipe is still full 



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: default avatarOleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/all/20241229135737.GA3293@redhat.com/
Link: https://lore.kernel.org/r/20250102140715.GA7091@redhat.com


Reported-by: default avatarWangYuli <wangyuli@uniontech.com>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent d2fc0ed5

fs/pipe.c

+10 −9
Original line number Diff line number Diff line
@@ -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);