Unverified Commit 478dbf12 authored by Max Kellermann's avatar Max Kellermann Committed by Christian Brauner
Browse files

fs/pipe: use spinlock in pipe_read() only if there is a watch_queue 



If there is no watch_queue, holding the pipe mutex is enough to
prevent concurrent writes, and we can avoid the spinlock.

O_NOTIFICATION_QUEUE is an exotic and rarely used feature, and of all
the pipes that exist at any given time, only very few actually have a
watch_queue, therefore it appears worthwile to optimize the common
case.

This patch does not optimize pipe_resize_ring() where the spinlocks
could be avoided as well; that does not seem like a worthwile
optimization because this function is not called often.

Related commits:

- commit 8df44129 ("pipe: Check for ring full inside of the
  spinlock in pipe_write()")
- commit b667b867 ("pipe: Advance tail pointer inside of wait
  spinlock in pipe_read()")
- commit 189b0ddc ("pipe: Fix missing lock in pipe_resize_ring()")

Signed-off-by: default avatarMax Kellermann <max.kellermann@ionos.com>
Message-Id: <20230921075755.1378787-4-max.kellermann@ionos.com>
Reviewed-by: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent dfaabf91

fs/pipe.c

+32 −11
Original line number Diff line number Diff line
@@ -227,6 +227,36 @@ static inline bool pipe_readable(const struct pipe_inode_info *pipe) 
	return !pipe_empty(head, tail) || !writers;

}



static inline unsigned int pipe_update_tail(struct pipe_inode_info *pipe,

					    struct pipe_buffer *buf,

					    unsigned int tail)

{

	pipe_buf_release(pipe, buf);



	/*

	 * If the pipe has a watch_queue, we need additional protection

	 * by the spinlock because notifications get posted with only

	 * this spinlock, no mutex

	 */

	if (pipe_has_watch_queue(pipe)) {

		spin_lock_irq(&pipe->rd_wait.lock);

#ifdef CONFIG_WATCH_QUEUE

		if (buf->flags & PIPE_BUF_FLAG_LOSS)

			pipe->note_loss = true;

#endif

		pipe->tail = ++tail;

		spin_unlock_irq(&pipe->rd_wait.lock);

		return tail;

	}



	/*

	 * Without a watch_queue, we can simply increment the tail

	 * without the spinlock - the mutex is enough.

	 */

	pipe->tail = ++tail;

	return tail;

}



static ssize_t

pipe_read(struct kiocb *iocb, struct iov_iter *to)

{
@@ -320,17 +350,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to) 
				buf->len = 0;

			}



			if (!buf->len) {

				pipe_buf_release(pipe, buf);

				spin_lock_irq(&pipe->rd_wait.lock);

#ifdef CONFIG_WATCH_QUEUE

				if (buf->flags & PIPE_BUF_FLAG_LOSS)

					pipe->note_loss = true;

#endif

				tail++;

				pipe->tail = tail;

				spin_unlock_irq(&pipe->rd_wait.lock);

			}

			if (!buf->len)

				tail = pipe_update_tail(pipe, buf, tail);

			total_len -= chars;

			if (!total_len)

				break;	/* common path: read succeeded */