From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933637AbbLVTex (ORCPT ); Tue, 22 Dec 2015 14:34:53 -0500 Received: from bh-25.webhostbox.net ([208.91.199.152]:43320 "EHLO bh-25.webhostbox.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933613AbbLVTev (ORCPT ); Tue, 22 Dec 2015 14:34:51 -0500 Subject: Re: [PATCH 2/5] watchdog: Separate and maintain variables based on variable lifetime To: Damien Riegel , linux-watchdog@vger.kernel.org, Wim Van Sebroeck , Pratyush Anand , Hans de Goede , linux-kernel@vger.kernel.org References: <1450645503-16661-1-git-send-email-linux@roeck-us.net> <1450645503-16661-3-git-send-email-linux@roeck-us.net> <20151221172815.GC12696@localhost> <5678A322.2010109@roeck-us.net> <20151222160907.GC6164@localhost> <567978D0.2060801@roeck-us.net> <20151222192807.GA16755@localhost> From: Guenter Roeck Message-ID: <5679A5D8.7020000@roeck-us.net> Date: Tue, 22 Dec 2015 11:34:48 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 MIME-Version: 1.0 In-Reply-To: <20151222192807.GA16755@localhost> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Authenticated_sender: linux@roeck-us.net X-OutGoing-Spam-Status: No, score=-1.0 X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - bh-25.webhostbox.net X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - roeck-us.net X-Get-Message-Sender-Via: bh-25.webhostbox.net: authenticated_id: linux@roeck-us.net X-Authenticated-Sender: bh-25.webhostbox.net: linux@roeck-us.net X-Source: X-Source-Args: X-Source-Dir: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/22/2015 11:28 AM, Damien Riegel wrote: > On Tue, Dec 22, 2015 at 08:22:40AM -0800, Guenter Roeck wrote: >> On 12/22/2015 08:09 AM, Damien Riegel wrote: >>> On Mon, Dec 21, 2015 at 05:10:58PM -0800, Guenter Roeck wrote: >>>> On 12/21/2015 09:28 AM, Damien Riegel wrote: >>>>> On Sun, Dec 20, 2015 at 01:05:00PM -0800, Guenter Roeck wrote: >>>>>> All variables required by the watchdog core to manage a watchdog are >>>>>> currently stored in struct watchdog_device. The lifetime of those >>>>>> variables is determined by the watchdog driver. However, the lifetime >>>>>> of variables used by the watchdog core differs from the lifetime of >>>>>> struct watchdog_device. To remedy this situation, watchdog drivers >>>>>> can implement ref and unref callbacks, to be used by the watchdog >>>>>> core to lock struct watchdog_device in memory. >>>>>> >>>>>> While this solves the immediate problem, it depends on watchdog drivers >>>>>> to actually implement the ref/unref callbacks. This is error prone, >>>>>> often not implemented in the first place, or not implemented correctly. >>>>>> >>>>>> To solve the problem without requiring driver support, split the variables >>>>>> in struct watchdog_device into two data structures - one for variables >>>>>> associated with the watchdog driver, one for variables associated with >>>>>> the watchdog core. With this approach, the watchdog core can keep track >>>>>> of its variable lifetime and no longer depends on ref/unref callbacks >>>>>> in the driver. As a side effect, some of the variables originally in >>>>>> struct watchdog_driver are now private to the watchdog core and no longer >>>>>> visible in watchdog drivers. >>>>>> >>>>>> The 'ref' and 'unref' callbacks in struct watchdog_driver are no longer >>>>>> used and marked as deprecated. >>>>> >>>>> Two comments below. It's great to see that unbinding a driver no longer >>>>> triggers a kernel panic. >>>>> >>>> It should not have caused a panic to start with, but the ref/unref functions >>>> for the most part were either not or wrongly implemented. Not really >>>> surprising - it took me a while to understand the problem. >>> >>> I tested on a driver which did not implement ref/unref. When ping is >>> called, it tries to dereference a freed 'struct watchdog_device' in >>> watchdog_get_drvdata, leading to a panic. >>> >> Yes, that will happen. Problem here is that the driver is buggy - >> pretty much all drivers which dynamically allocate struct watchdog_device >> have this problem. >> >> This is the ultimate reason for coming up with this patch. >> >>>> >>>> [ ... ] >>>> >>>>>> >>>>>> /* >>>>>> + * struct _watchdog_device - watchdog core internal data >>>>> >>>>> Think it should be /**. Anyway, I find it confusing to have both >>>>> _watchdog_device and watchdog_device, but I can't think of a better >>>>> name right now. >>>> >>>> I renamed the data structure to watchdog_data and moved it into watchdog_dev.c >>>> since it is only used there. No '**', though, because it is not a published >>>> API, but just an internal data structure. >>>> >>>> I also renamed the matching variable name to 'wd_data' (from '_wdd'). >>> >>> Okay. Also, why didn't you use the explicit type for 'wdd_data' in >>> 'struct watchdog_device' instead of a void*? >>> >> >> This is to hide the data type, since the structure is not exported >> to drivers. >> >> I could pre-declare the structure with 'struct watchdog_data;', but >> then I'd have to use a different structure name (watchdog_cdev_data, >> maybe, or watchdog_core_data) to make it less generic. Any opinion ? >> Would that be better / preferred ? I am 50/50 about it. > > My personal preference would be to be explicit. That makes code > nagivation easier and it might help the compiler catch some mistakes. > Plus, as you moved the structure definition in watchdog_dev.c, it is > very clear that drivers aren't supposed to use it. > Ok, makes sense. I'll do that. Guenter