[PATCH] io_uring/rw: do not infer NOWAIT support from poll readiness

[Tianyi Chen <hi@tychen.cc>]

A poll-ready file can still block if another reader consumes the data or
a write exceeds the available space. Without FMODE_NOWAIT or O_NONBLOCK,
IOCB_NOWAIT does not prevent this from blocking the ring submitter.

Remove the poll fallback and route unsupported requests to io-wq after
polling, rather than repeatedly retrying them in task work. Downgrade
multishot reads on these files to single-shot completion; files with
nonblocking support keep multishot behavior.

Fixes: f7c913438533 ("io_uring/rw: allow pollable non-blocking attempts for !FMODE_NOWAIT")

Closes: https://github.com/axboe/liburing/issues/1623

Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
---
 io_uring/rw.c | 25 ++++++++-----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

diff --git a/io_uring/rw.c b/io_uring/rw.c
index 432820f86251..0c3585f8d5f0 100644
--- a/io_uring/rw.c
+++ b/io_uring/rw.c
@@ -34,21 +34,6 @@ struct io_rw {
 	rwf_t				flags;
 };
 
-static bool io_file_supports_nowait(struct io_kiocb *req, __poll_t mask)
-{
-	/* If FMODE_NOWAIT is set for a file, we're golden */
-	if (req->flags & REQ_F_SUPPORT_NOWAIT)
-		return true;
-	/* No FMODE_NOWAIT, if we can poll, check the status */
-	if (io_file_can_poll(req)) {
-		struct poll_table_struct pt = { ._key = mask };
-
-		return vfs_poll(req->file, &pt) & mask;
-	}
-	/* No FMODE_NOWAIT support, and file isn't pollable. Tough luck. */
-	return false;
-}
-
 static int io_iov_buffer_select_prep(struct io_kiocb *req)
 {
 	struct iovec __user *uiov;
@@ -859,6 +844,10 @@ static int io_rw_init_file(struct io_kiocb *req, fmode_t mode, int rw_type)
 	if (!(req->flags & REQ_F_FIXED_FILE))
 		req->flags |= io_file_get_flags(file);
 
+	/* Poll readiness cannot guarantee that a read or write will not block. */
+	if (!(req->flags & REQ_F_SUPPORT_NOWAIT))
+		req->flags |= REQ_F_FORCE_ASYNC;
+
 	kiocb->ki_flags = file->f_iocb_flags;
 	ret = kiocb_set_rw_flags(kiocb, rw->flags, rw_type);
 	if (unlikely(ret))
@@ -936,7 +925,7 @@ static int __io_read(struct io_kiocb *req, struct io_br_sel *sel,
 
 	if (force_nonblock) {
 		/* If the file doesn't support async, just async punt */
-		if (unlikely(!io_file_supports_nowait(req, EPOLLIN)))
+		if (unlikely(!(req->flags & REQ_F_SUPPORT_NOWAIT)))
 			return -EAGAIN;
 		kiocb->ki_flags |= IOCB_NOWAIT;
 	} else {
@@ -1055,6 +1044,8 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 	/* make it sync, multishot doesn't support async execution */
 	rw->kiocb.ki_complete = NULL;
 	ret = __io_read(req, &sel, issue_flags);
+	if (!(req->flags & REQ_F_SUPPORT_NOWAIT))
+		req->flags &= ~REQ_F_APOLL_MULTISHOT;
 
 	/*
 	 * If we get -EAGAIN, recycle our buffer and just let normal poll
@@ -1149,7 +1140,7 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
 
 	if (force_nonblock) {
 		/* If the file doesn't support async, just async punt */
-		if (unlikely(!io_file_supports_nowait(req, EPOLLOUT)))
+		if (unlikely(!(req->flags & REQ_F_SUPPORT_NOWAIT)))
 			goto ret_eagain;
 
 		/* Check if we can support NOWAIT. */
-- 
2.55.0