Re: [PATCH 0/1] pipe: only enable the extra wake_up(rd_wait) when epoll is actually used

[Oleg Nesterov <oleg@redhat.com>]

On 07/23, Oleg Nesterov wrote:
>
> OK, sashiko has some concerns
>
> 	https://sashiko.dev/#/patchset/amIqmbbZx3NlzUsX%40redhat.com

Let me quote:

	Does skipping this wakeup for non-epoll consumers break io_uring?

	Applications polling pipes via io_uring do not attach an eventpoll context,
	so pipe->epoll_usage will be false. If a writer writes to an empty pipe,
	io_uring receives the wakeup.

	If the writer then writes a second chunk before the first is drained,
	anon_pipe_write() observes was_empty == false and
	pipe_get_epoll_usage() == false, skipping the waitqueue wakeup.

	Could this cause io_uring to miss events and hang permanently, waiting
	for a CQE that will never be emitted for the new data?

and I am starting to think sashiko is right (damn as always ;) and this
patch does affect/break io_uring.

Jens, could you confirm? If yes, we need to update the comments in pipe.c
(I've attached 1/1 at the end, so that you can see what this patch does)

I know nothing about io_uring and io_uring/poll.c is not trivial to say at
least ;) please check my understanding.

It seems that IORING_OP_POLL_ADD / IORING_POLL_ADD_MULTI is edge-triggered
by default! Like EPOLL_CTL_ADD / EPOLLET.

This means that io_uring depends on the "nasty semantics" too, io_poll_wake()
path should add the task work which calls io_req_post_cqe() every time the new
data arrives, even if the pipe was not empty.

Strange...

And. io_poll_parse_events() doesn't set EPOLLET if IORING_POLL_ADD_LEVEL, but
how is it possible to use IORING_POLL_ADD_LEVEL? io_poll_add_prep() only allows
IORING_POLL_ADD_MULTI in flags? OK, I don't understand this code anyway...

Oleg.
---

Subject: [PATCH] pipe: only enable the extra wake_up(rd_wait) when epoll is actually used

pipe_poll() unconditionally sets poll_usage on the first call, forcing
anon_pipe_write() to wake up readers on every write even if the pipe was
not empty. But this is only needed for epoll's "nasty semantics"; poll()
and select() users pay for it for no reason.

Rename it to epoll_usage, and only set it when the caller is actually
using epoll on the read side of the pipe.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/pipe.c                 | 23 +++++++++++++++++++----
 include/linux/pipe_fs_i.h |  6 ++++--
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 32140cb00d7e..b45b4b8d0b0b 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -357,6 +357,23 @@ static inline unsigned int pipe_update_tail(struct pipe_inode_info *pipe,
 	return tail;
 }
 
+static void pipe_set_epoll_usage(struct file *filp, struct pipe_inode_info *pipe)
+{
+#ifdef CONFIG_EPOLL
+	if ((filp->f_mode & FMODE_READ) && filp->f_ep &&
+	    unlikely(!READ_ONCE(pipe->epoll_usage)))
+		WRITE_ONCE(pipe->epoll_usage, true);
+#endif
+}
+
+static bool pipe_get_epoll_usage(struct pipe_inode_info *pipe)
+{
+#ifdef CONFIG_EPOLL
+	return pipe->epoll_usage;
+#endif
+	return false;
+}
+
 static ssize_t
 anon_pipe_read(struct kiocb *iocb, struct iov_iter *to)
 {
@@ -689,7 +706,7 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	 * Epoll nonsensically wants a wakeup whether the pipe
 	 * was already empty or not.
 	 */
-	if (was_empty || pipe->poll_usage)
+	if (was_empty || pipe_get_epoll_usage(pipe))
 		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
 	kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 	if (wake_next_writer)
@@ -761,9 +778,7 @@ pipe_poll(struct file *filp, poll_table *wait)
 	union pipe_index idx;
 
 	/* Epoll has some historical nasty semantics, this enables them */
-	if (unlikely(!READ_ONCE(pipe->poll_usage)))
-		WRITE_ONCE(pipe->poll_usage, true);
-
+	pipe_set_epoll_usage(filp, pipe);
 	/*
 	 * Reading pipe state only -- no need for acquiring the semaphore.
 	 *
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 7f6a92ac9704..d06da9bad3a8 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -74,7 +74,7 @@ union pipe_index {
  *	@files: number of struct file referring this pipe (protected by ->i_lock)
  *	@r_counter: reader counter
  *	@w_counter: writer counter
- *	@poll_usage: is this pipe used for epoll, which has crazy wakeups?
+ *	@epoll_usage: is this pipe used for epoll, which has crazy wakeups?
  *	@fasync_readers: reader side fasync
  *	@fasync_writers: writer side fasync
  *	@bufs: the circular array of pipe buffers
@@ -95,7 +95,9 @@ struct pipe_inode_info {
 	unsigned int files;
 	unsigned int r_counter;
 	unsigned int w_counter;
-	bool poll_usage;
+#ifdef CONFIG_EPOLL
+	bool epoll_usage;
+#endif
 #ifdef CONFIG_WATCH_QUEUE
 	bool note_loss;
 #endif
-- 
2.52.0