From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8E16C43217 for ; Sun, 20 Nov 2022 20:46:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229633AbiKTUqb (ORCPT ); Sun, 20 Nov 2022 15:46:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38330 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229498AbiKTUq3 (ORCPT ); Sun, 20 Nov 2022 15:46:29 -0500 Received: from netrider.rowland.org (netrider.rowland.org [192.131.102.5]) by lindbergh.monkeyblade.net (Postfix) with SMTP id 1034EE0CC for ; Sun, 20 Nov 2022 12:46:27 -0800 (PST) Received: (qmail 99007 invoked by uid 1000); 20 Nov 2022 15:46:26 -0500 Date: Sun, 20 Nov 2022 15:46:26 -0500 From: Alan Stern To: John Keeping Cc: Lee Jones , Greg KH , balbi@kernel.org, linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org Subject: Re: [PATCH 1/1] usb: gadget: f_hid: Conduct proper refcounting on shared f_hidg pointer Message-ID: References: <20221117120813.1257583-1-lee@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 20, 2022 at 05:22:19PM +0000, John Keeping wrote: > On Fri, Nov 18, 2022 at 04:07:24PM -0500, Alan Stern wrote: > > On Fri, Nov 18, 2022 at 04:37:32PM +0000, John Keeping wrote: > > > I don't think it's at all simple to fix this - I posted a series > > > addressing the lifetime issues here a few years ago but didn't chase it > > > up and there was no feedback: > > > > > > https://lore.kernel.org/linux-usb/20191028114228.3679219-1-john@metanate.com/ > > > > > > That includes a patch to remove the embedded struct cdev and manage its > > > lifetime separately, which I think is needed as there are two different > > > struct device objects here and we cannot tie their lifetimes together. > > > > I still don't have a clear picture of what the real problem is. Lee's > > original patch description just said "external references are presently > > not tracked", with no details about what those external references are. > > Why not add just proper cdev_get() and cdev_put() calls to whatever code > > handles those external references, so that they _are_ tracked? > > > > What are the two different struct device objects? Why do their > > lifetimes need to be tied together? If you do need to tie their > > lifetimes somehow, why not simply make one of them (the one which is > > logically allowed to be shorter-lived) hold a reference to the other? > > The problem is that we have a struct cdev embedded in f_hidg but the > lifetime of f_hidg is not tied to any kobject so we can't solve this in > the right way by setting the parent kobject of the cdev. > > While refcounting struct f_hidg is necessary, it's not sufficient > because the only way to keep it alive long enough for the final > kobject_put() on the embedded cdev is to tie the lifetime to a kobject > of its own and there is no suitable object as this is not the model > followed by gadget function instances. I see. The solution is simple: Embed a struct device in struct f_hidg, and call cdev_device_add() to add the device and the cdev. This will automatically make the device the parent of the cdev, so the device's refcount won't go to 0 until the cdev's refcount does. Then you can tie the f_hidg's lifetime to the device's, so the device's release routine can safely deallocate the entire f_hidg structure. The parent of the new struct device should be set to &gadget->dev. If you can't think of a better name for the device, you could simply append ":I" to the parent's name, where I is the interface number, or even append ":C.I" where C is the config number (like we do on the host side). Alan Stern