[PATCH] io_uring/sqpoll: don't set IO_SQ_THREAD_SHOULD_STOP on thread exit

[Shuai Liu <847807867@qq.com>]

io_sq_thread_stop() carries a WARN_ON_ONCE() asserting that
IO_SQ_THREAD_SHOULD_STOP isn't set yet, i.e. that stopping the SQPOLL
thread is never attempted twice. Since commit 22d9db9fd64a
("io_uring/sqpoll: don't attach to a thread that is canceling"), the
SQPOLL thread itself sets that bit on its way out, so that
io_sq_offload_create() won't attach a new ring to a thread that is
canceling.

When the thread exits on a fatal signal, the bit is thus set before the
ring teardown gets to io_sq_thread_stop(), which now triggers the
warning on every such exit:

  WARNING: io_uring/sqpoll.c:88 at io_sq_thread_stop
  Workqueue: iou_exit io_ring_exit_work
  Call Trace:
   io_put_sq_data
   io_sq_thread_finish
   io_ring_ctx_free
   io_ring_exit_work

Introduce a separate IO_SQ_THREAD_EXITING bit for the thread marking
itself as on its way out, and check it in io_sq_offload_create() along
with IO_SQ_THREAD_SHOULD_STOP. IO_SQ_THREAD_SHOULD_STOP regains its
original meaning, "a stop has been requested", and the WARN_ON_ONCE()
holds again.

Reproduced with an SQPOLL ring and an immediate fatal signal: the
warning fires on v7.3.0+ (22d9db9fd64a present) and is gone with this
change.

Reported-by: syzbot+675af112519ca34f5502@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=675af112519ca34f5502
Fixes: 22d9db9fd64a ("io_uring/sqpoll: don't attach to a thread that is canceling")
Signed-off-by: Shuai Liu <847807867@qq.com>
---
 io_uring/sqpoll.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index c833e776320d..bbfb1a6e6668 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -29,6 +29,7 @@
 enum {
 	IO_SQ_THREAD_SHOULD_STOP = 0,
 	IO_SQ_THREAD_SHOULD_PARK,
+	IO_SQ_THREAD_EXITING,
 };
 
 void io_sq_thread_unpark(struct io_sq_data *sqd)
@@ -436,9 +437,12 @@ static int io_sq_thread(void *data)
 	/*
 	 * Cancelation drops sqd->lock while it waits, and a ring could attach
 	 * to a thread that is on its way out. A fatal signal doesn't set the
-	 * bit, so set it here for io_sq_offload_create() to see.
+	 * stop bit, so mark the thread as exiting here for
+	 * io_sq_offload_create() to see. IO_SQ_THREAD_SHOULD_STOP is left
+	 * alone, as it means a stop has been requested via
+	 * io_sq_thread_stop().
 	 */
-	set_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd->state);
+	set_bit(IO_SQ_THREAD_EXITING, &sqd->state);
 	io_uring_cancel_generic(true, sqd);
 	rcu_assign_pointer(sqd->thread, NULL);
 	put_task_struct(current);
@@ -510,7 +514,8 @@ __cold int io_sq_offload_create(struct io_ring_ctx *ctx,
 		/* don't attach to a dying SQPOLL thread, would be racy */
 		ret = 0;
 		if (attached && (!sqd->thread ||
-		    test_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd->state)))
+		    test_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd->state) ||
+		    test_bit(IO_SQ_THREAD_EXITING, &sqd->state)))
 			ret = -ENXIO;
 		io_sq_thread_unpark(sqd);
 
-- 
2.43.0