io_uring: ensure deferred completions are posted for multishot

Commit 687b2bae0efff9b25e071737d6af5004e6e35af5 upstream.

Multishot normally uses io_req_post_cqe() to post completions, but when
stopping it, it may finish up with a deferred completion. This is fine,
except if another multishot event triggers before the deferred completions
get flushed. If this occurs, then CQEs may get reordered in the CQ ring,
and cause confusion on the application side.

When multishot posting via io_req_post_cqe(), flush any pending deferred
completions first, if any.

Cc: [email protected] # 6.1+
Reported-by: Norman Maurer <[email protected]>
Reported-by: Christian Mazakas <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: axboe

io_uring/io_uring.c

@@ -819,6 +819,14 @@ bool io_post_aux_cqe(struct io_ring_ctx *ctx,
 {
 	bool filled;
 
+	/*
+	 * If multishot has already posted deferred completions, ensure that
+	 * those are flushed first before posting this one. If not, CQEs
+	 * could get reordered.
+	 */
+	if (!wq_list_empty(&ctx->submit_state.compl_reqs))
+		__io_submit_flush_completions(ctx);
+
 	io_cq_lock(ctx);
 	filled = io_fill_cqe_aux(ctx, user_data, res, cflags, allow_overflow);
 	io_cq_unlock_post(ctx);