* Re: [PATCH 0/1] pipe: only enable the extra wake_up(rd_wait) when epoll is actually used
[not found] ` <amIu5WWgTNkdvqIz@redhat.com>
@ 2026-07-24 13:58 ` Oleg Nesterov
2026-07-24 14:43 ` Mateusz Guzik
0 siblings, 1 reply; 3+ messages in thread
From: Oleg Nesterov @ 2026-07-24 13:58 UTC (permalink / raw)
To: Breno Leitao, Christian Brauner, Mateusz Guzik, Jens Axboe
Cc: Alexander Viro, Jan Kara, linux-fsdevel, linux-kernel, io-uring,
Alexey Gladkov
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
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