io_uring: remove io_free_req_tw

Request completion is a very hot path in general, but there are 3 places
that can be doing it: io_free_batch_list(), io_req_complete_post() and
io_free_req_tw().

io_free_req_tw() is used rather marginally and we don't care about it.
Killing it can help to clean up and optimise the left two, do that by
replacing it with io_req_task_complete().

There are two things to consider:
1) io_free_req() is called when all refs are put, so we need to reinit
   references. The easiest way to do that is to clear REQ_F_REFCOUNT.
2) We also don't need a cqe from it, so silence it with REQ_F_CQE_SKIP.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/434a2be8f33d474ad888ce1c17fe5ea7bbcb2a55.1687518903.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/io_uring.c

@@ -1121,26 +1121,13 @@ static inline void io_dismantle_req(struct io_kiocb *req)
 		io_put_file(req->file);
 }
 
-static __cold void io_free_req_tw(struct io_kiocb *req, struct io_tw_state *ts)
-{
-	struct io_ring_ctx *ctx = req->ctx;
-
-	if (req->rsrc_node) {
-		io_tw_lock(ctx, ts);
-		io_put_rsrc_node(ctx, req->rsrc_node);
-	}
-	io_dismantle_req(req);
-	io_put_task_remote(req->task, 1);
-
-	spin_lock(&ctx->completion_lock);
-	wq_list_add_head(&req->comp_list, &ctx->locked_free_list);
-	ctx->locked_free_nr++;
-	spin_unlock(&ctx->completion_lock);
-}
-
 __cold void io_free_req(struct io_kiocb *req)
 {
-	req->io_task_work.func = io_free_req_tw;
+	/* refs were already put, restore them for io_req_task_complete() */
+	req->flags &= ~REQ_F_REFCOUNT;
+	/* we only want to free it, don't post CQEs */
+	req->flags |= REQ_F_CQE_SKIP;
+	req->io_task_work.func = io_req_task_complete;
 	io_req_task_work_add(req);
 }