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 578A7C761AF for ; Sat, 1 Apr 2023 15:21:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229379AbjDAPV2 (ORCPT ); Sat, 1 Apr 2023 11:21:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55212 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229570AbjDAPVZ (ORCPT ); Sat, 1 Apr 2023 11:21:25 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CA40024AFD; Sat, 1 Apr 2023 08:21:09 -0700 (PDT) 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 7A59AB80BA5; Sat, 1 Apr 2023 15:21:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7F3C6C433EF; Sat, 1 Apr 2023 15:21:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1680362467; bh=VRDENP0GBG7HEPJPomXEJE7ipvrog3G07035r41JCJ8=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=h1+F7BuKL9lM30A5/t9JaKr/WJreYYGl8tgMNUJufsHzv/7TyX0pJeaZ4GqmVlE8o 8yo1OhgVT9lsepFxu2sKxvbn1ya2ityYCNWj4Tr/RBsIdH404gWZ+kSBdvyDN1EmgN KAA0gJLuuTiC9ZXvovGFK2/rNXfzpv0XLBQI4JhnT6N+Kycbekeg4Da4bjjmJ1SyH4 DKDj4TpO+HLmzJ97joKmDPt5u5bP3rS3uU9zbbDIQYQlUeySGbCvDZ9PPmTHs9of+Y yEQLlHS3xMX+MkvCDHITjKXTsPHNDSNOpVp6WePlDwGoOZ8axw2XyK0WiTVSq4gaao vQX8+PmsdI+Xw== Date: Sat, 1 Apr 2023 16:36:14 +0100 From: Jonathan Cameron To: Maxime Ripard Cc: Matti Vaittinen , Greg Kroah-Hartman , Matti Vaittinen , "Rafael J. Wysocki" , Brendan Higgins , David Gow , Andy Shevchenko , Heikki Krogerus , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, kunit-dev@googlegroups.com, Stephen Boyd , linux-iio@vger.kernel.org, Lars-Peter Clausen Subject: Re: [PATCH v5 1/8] drivers: kunit: Generic helpers for test device creation Message-ID: <20230401163614.38f68397@jic23-huawei> In-Reply-To: <20230329194609.7u2hgidxdk34emsf@penduick> References: <20230323101216.w56kz3rudlj23vab@houat> <8a03a6fb-39b9-cd17-cc10-ece71111357d@gmail.com> <20230323122925.kqdnomr7i46qnyo4@houat> <590189b3-42d9-ab12-fccd-37338595cb6f@gmail.com> <20230323163639.xtwpid2uunwnzai4@houat> <20230324123157.bbwvfq4gsxnlnfwb@houat> <20230325175044.7bee9e7d@jic23-huawei> <20230329194609.7u2hgidxdk34emsf@penduick> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.37; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org On Wed, 29 Mar 2023 21:46:09 +0200 Maxime Ripard wrote: > On Sat, Mar 25, 2023 at 05:50:44PM +0000, Jonathan Cameron wrote: > > On Fri, 24 Mar 2023 13:31:57 +0100 > > Maxime Ripard wrote: > > > > > On Fri, Mar 24, 2023 at 08:11:52AM +0200, Matti Vaittinen wrote: > > > > On 3/23/23 18:36, Maxime Ripard wrote: > > > > > On Thu, Mar 23, 2023 at 03:02:03PM +0200, Matti Vaittinen wrote: > > > > > > On 3/23/23 14:29, Maxime Ripard wrote: > > > > > > > On Thu, Mar 23, 2023 at 02:16:52PM +0200, Matti Vaittinen wrote: > > > > > > > > > > > > > > This is the description of what was happening: > > > > > > > https://lore.kernel.org/dri-devel/20221117165311.vovrc7usy4efiytl@houat/ > > > > > > > > > > > > Thanks Maxime. Do I read this correcty. The devm_ unwinding not being done > > > > > > when root_device_register() is used is not because root_device_unregister() > > > > > > would not trigger the unwinding - but rather because DRM code on top of this > > > > > > device keeps the refcount increased? > > > > > > > > > > There's a difference of behaviour between a root_device and any device > > > > > with a bus: the root_device will only release the devm resources when > > > > > it's freed (in device_release), but a bus device will also do it in > > > > > device_del (through bus_remove_device() -> device_release_driver() -> > > > > > device_release_driver_internal() -> __device_release_driver() -> > > > > > device_unbind_cleanup(), which are skipped (in multiple places) if > > > > > there's no bus and no driver attached to the device). > > > > > > > > > > It does affect DRM, but I'm pretty sure it will affect any framework > > > > > that deals with device hotplugging by deferring the framework structure > > > > > until the last (userspace) user closes its file descriptor. So I'd > > > > > assume that v4l2 and cec at least are also affected, and most likely > > > > > others. > > > > > > > > Thanks for the explanation and patience :) > > > > > > > > > > > > > > > If this is the case, then it sounds like a DRM specific issue to me. > > > > > > > > > > I mean, I guess. One could also argue that it's because IIO doesn't > > > > > properly deal with hotplugging. > > > > > > > > I must say I haven't been testing the IIO registration API. I've only tested > > > > the helper API which is not backed up by any "IIO device". (This is fine for > > > > the helper because it must by design be cleaned-up only after the > > > > IIO-deregistration). > > > > > > > > After your explanation here, I am not convinced IIO wouldn't see the same > > > > issue if I was testing the devm_iio_device_alloc() & co. > > > > > > It depends really. The issue DRM is trying to solve is that, when a > > > device is gone, some application might still have an open FD and could > > > still poke into the kernel, while all the resources would have been > > > free'd if it was using devm. > > > > > > So everything is kept around until the last fd is closed, so you still > > > have a reference to the device (even though it's been removed from its > > > bus) until that time. > > > > > > It could be possible that IIO just doesn't handle that case at all. I > > > guess most of the devices aren't hotpluggable, and there's not much to > > > interact with from a userspace PoV iirc, so it might be why. > > > > Lars-Peter Clausen (IIRC) fixed up the IIO handling of the similar cases a > > long time ago now. It's simpler that for some other subsystems as we don't > > have as many interdependencies as occur in DRM etc. > > > > I 'think' we are fine in general with the IIO approach to this (I think we > > did have one report of a theoretical race condition in the remove path that > > was never fully addressed). > > > > For IIO we also have fds that can be open but all accesses to them are proxied > > through the IIO core and one of the things iio_device_unregister() or the devm > > equivalent does is to set indio_dev->info = NULL (+ wake up anyone waiting on > > data etc). Alongside removing the callbacks, that is also used as a flag > > to indicate the device has gone. > > Sorry if it came as trying to put IIO under a bad light, it certainly > wasn't my intention. I was trying to come up with possible explanations > as to why IIO's design was simpler than DRM is :) No problem :) I'm sure there are gremlins hiding there. Part of the problem is that nothing prevents drivers doing 'wrong things' other than us noticing when it happens. > > > Note that we keep a reference to the struct indio_dev->dev (rather that the > > underlying device) so that is not freed until the last fd is closed. > > Thus, although devm unwinding has occurred that doesn't mean all the data > > that was allocated with devm_xx calls is cleared up immediately. > > I'm not sure I get that part though. devm unwinding can happen even if the refcount is > 1 No IIO driver should be using devm on the indio_dev->dev, they should be doing it on the parent device. When the devm_iio_device_free() gets called, that doesn't actually free the device. Just decrements a reference count (earlier on we already ensured that it is just acting as a stub that provides no access to the underlying device for open FDs.). There are probably more problems hiding though! Jonathan > > Maxime