io_uring/rsrc: improve code generation for fixed file assignment

For the normal read/write path, we have already locked the ring
submission side when assigning the file. This causes branch
mispredictions when we then check and try and lock again in
io_req_set_rsrc_node(). As this is a very hot path, this matters.

Add a basic helper that already assumes we already have it locked,
and use that in io_file_get_fixed().
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -2000,9 +2000,10 @@ inline struct file *io_file_get_fixed(struct io_kiocb *req, int fd,
 		goto out;
 	fd = array_index_nospec(fd, ctx->nr_user_files);
 	slot = io_fixed_file_slot(&ctx->file_table, fd);
-	file = io_slot_file(slot);
+	if (!req->rsrc_node)
+		__io_req_set_rsrc_node(req, ctx);
 	req->flags |= io_slot_flags(slot);
-	io_req_set_rsrc_node(req, ctx, 0);
+	file = io_slot_file(slot);
 out:
 	io_ring_submit_unlock(ctx, issue_flags);
 	return file;

io_uring/rsrc.h

@@ -102,17 +102,21 @@ static inline void io_charge_rsrc_node(struct io_ring_ctx *ctx,
 	node->refs++;
 }
 
+static inline void __io_req_set_rsrc_node(struct io_kiocb *req,
+					  struct io_ring_ctx *ctx)
+{
+	lockdep_assert_held(&ctx->uring_lock);
+	req->rsrc_node = ctx->rsrc_node;
+	io_charge_rsrc_node(ctx, ctx->rsrc_node);
+}
+
 static inline void io_req_set_rsrc_node(struct io_kiocb *req,
 					struct io_ring_ctx *ctx,
 					unsigned int issue_flags)
 {
 	if (!req->rsrc_node) {
 		io_ring_submit_lock(ctx, issue_flags);
-
-		lockdep_assert_held(&ctx->uring_lock);
-
-		req->rsrc_node = ctx->rsrc_node;
-		io_charge_rsrc_node(ctx, ctx->rsrc_node);
+		__io_req_set_rsrc_node(req, ctx);
 		io_ring_submit_unlock(ctx, issue_flags);
 	}
 }