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 922D2C433FE for ; Mon, 21 Nov 2022 12:32:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230401AbiKUMcY (ORCPT ); Mon, 21 Nov 2022 07:32:24 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46754 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230145AbiKUMcD (ORCPT ); Mon, 21 Nov 2022 07:32:03 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 41212B4804; Mon, 21 Nov 2022 04:30:53 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id A00FEB80F02; Mon, 21 Nov 2022 12:30:51 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 85B8FC433D6; Mon, 21 Nov 2022 12:30:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1669033850; bh=jckmrG5WyHMue6fgjekmw1q+D7Nj07UdM99iFtf76Zk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=GBBYM662aPITuBqPbRrmAeHsvtTMEuJ24Ek3b6zSpUFIrtWaxv8OBPnCip5wftHf1 SzhQJkhq7JHFgfRayMPQ+DTn4XF9qNydHqa87Pu3cYO8qdhCZ8/vOIv6Dt/nS18xPA RAYDUfXeZX8BVYS5U99LlGPqbfk0FUKwpdWhGp2XIOiTw+g9OupcWcg7vC4FtNlcdm bLXqJRg3nbSLr0XyRWrqYS8IFb8Gn4+G+PdnGVjHKljens/kgBYHfYYzKYKy+BglB1 mYkbbyfxuVuibOZQ4KzziC4HixhGFMCM/3bZntETFhi40BYgnLuvq3JfvcBQYp4Hzc MlgI4OD0YtMgQ== Date: Mon, 21 Nov 2022 12:30:45 +0000 From: Lee Jones To: Alan Stern Cc: John Keeping , 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 20 Nov 2022, Alan Stern wrote: > 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). Thanks for the suggestions Alan. Not long finished speaking with Greg about this. He seems okay with the approach. I'll knock something together and get a "v1" (*wink*) out shortly. -- Lee Jones [李琼斯]