[PATCH AUTOSEL 6.12 7/7] io_uring: always do atomic put from iowq

Sasha Levin
2025-04-07T18:17:34Z

From: Pavel Begunkov <asml.silence@gmail.com>

[ Upstream commit 390513642ee6763c7ada07f0a1470474986e6c1c ]

io_uring always switches requests to atomic refcounting for iowq
execution before there is any parallilism by setting REQ_F_REFCOUNT,
and the flag is not cleared until the request completes. That should be
fine as long as the compiler doesn't make up a non existing value for
the flags, however KCSAN still complains when the request owner changes
oter flag bits:

BUG: KCSAN: data-race in io_req_task_cancel / io_wq_free_work
...
read to 0xffff888117207448 of 8 bytes by task 3871 on cpu 0:
req_ref_put_and_test io_uring/refs.h:22 [inline]

Skip REQ_F_REFCOUNT checks for iowq, we know it's set.

Reported-by: syzbot+903a2ad71fb3f1e47cf5@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d880bc27fb8c3209b54641be4ff6ac02b0e5789a.1743679736.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
io_uring/io_uring.c | 2 +-
io_uring/refs.h | 7 +++++++
2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index cf28d29fffbf0..20b6a9ec9da70 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1777,7 +1777,7 @@ struct io_wq_work *io_wq_free_work(struct io_wq_work *work)
struct io_kiocb *req = container_of(work, struct io_kiocb, work);
struct io_kiocb *nxt = NULL;

- if (req_ref_put_and_test(req)) {
+ if (req_ref_put_and_test_atomic(req)) {
if (req->flags & IO_REQ_LINK_FLAGS)
nxt = io_req_find_next(req);
io_free_req(req);
diff --git a/io_uring/refs.h b/io_uring/refs.h
index 63982ead9f7da..0d928d87c4ed1 100644
--- a/io_uring/refs.h
+++ b/io_uring/refs.h
@@ -17,6 +17,13 @@ static inline bool req_ref_inc_not_zero(struct io_kiocb *req)
return atomic_inc_not_zero(&req->refs);
}

+static inline bool req_ref_put_and_test_atomic(struct io_kiocb *req)
+{
+ WARN_ON_ONCE(!(data_race(req->flags) & REQ_F_REFCOUNT));
+ WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+ return atomic_dec_and_test(&req->refs);
+}
+
static inline bool req_ref_put_and_test(struct io_kiocb *req)
{
if (likely(!(req->flags & REQ_F_REFCOUNT)))
--
2.39.5