io_uring: restore RCU read section in io_req_local_work_add()

The task-work refactor that moved io_req_local_work_add() out of
io_uring.c into the new io_uring/tw.c dropped the whole-body
guard(rcu)() that used to cover the function body.

For DEFER_TASKRUN rings the ring teardown still relies on that RCU read
section pairing with its grace period:

	/* pairs with RCU read section in io_req_local_work_add() */
	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
		synchronize_rcu();
	io_ring_ctx_free(ctx);

io_req_local_work_add() keeps dereferencing ctx after mpscq_push() has
published the request to the work list (ctx->cq_wait_nr, and
ctx->submitter_task in the final wake_up_state()), without holding a ctx
reference across that window. The RCU read section was the only thing
guaranteeing an in-flight adder had finished touching ctx before
io_ring_ctx_free() ran; synchronize_rcu() only waits for readers that
are actually inside an RCU read-side critical section. With the guard
gone the grace period no longer pairs with anything on the add side, so
ctx can be freed and reused while io_req_local_work_add() is still using
it.

Fixes: d46ab2c ("io_uring: switch local task_work to a mpscq")
Signed-off-by: Woraphat Khiaodaeng <worapat.kd2@gmail.com>
Link: https://patch.msgid.link/20260709035100.2269-1-worapat.kd2@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Wrptt3885

io_uring/tw.c

@@ -153,6 +153,9 @@ void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
 	struct io_ring_ctx *ctx = req->ctx;
 	int nr_wait;
 
+	/* pairs with synchronize_rcu() in io_ring_exit_work() */
+	guard(rcu)();
+
 	/*
 	 * We don't know how many requests there are in the link and whether
 	 * they can even be queued lazily, fall back to non-lazy.