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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EBBF4C433FE for ; Wed, 6 Oct 2021 19:51:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CCD38610C7 for ; Wed, 6 Oct 2021 19:51:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239448AbhJFTxP (ORCPT ); Wed, 6 Oct 2021 15:53:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56970 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229892AbhJFTxN (ORCPT ); Wed, 6 Oct 2021 15:53:13 -0400 Received: from mail-wr1-x434.google.com (mail-wr1-x434.google.com [IPv6:2a00:1450:4864:20::434]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 29608C061746 for ; Wed, 6 Oct 2021 12:51:21 -0700 (PDT) Received: by mail-wr1-x434.google.com with SMTP id k7so12018126wrd.13 for ; Wed, 06 Oct 2021 12:51:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=Ai+3e+yiFoC0J7qCJE8CRMoYpaETZPv1B+QsKQNKBxU=; b=vq92d5MG8j8llq6Xph+KAv7K4wXgj0wJmuOKotV38cwtMqtlgj4lWKv/JntIbU5EeK Hn5l1OQ8Qo/+Nt34hF9ra5wLzYJJfOeE+kvUh89usr5JpWK2TkFyV3iPYR+Zx5NhrWb1 NkPOALpBL2vYCt6DfKuDV4HJCTE0n2UDBSYPbldzzZeIHf9HtY+bbxMRfE8gB57m0g2t 8BIfqB2gOfOEN4b/zyQkf0PvlN/xKK4WTUCGktDUwUWNpxCNo/RPOwpzegZ4TrSU/yMV DDqtyIWtCzKS/WFS0d9jGsFPBZ3iZSeOl3FSqW50nQpcmcYqZiDz0rHymhHG/JJeqZnq jQSw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=Ai+3e+yiFoC0J7qCJE8CRMoYpaETZPv1B+QsKQNKBxU=; b=JFnOJh6U6pXrro7WPzz3ytGchVmYZx09Odfqi4mR9ukaI4aJDKhYMvyK8F/edXwauv TezgWId6C1RWlHomwZnS/Sgdf72t88F4JRe8hLIUph5kSir54a3rs5xj2E6qr8a/f8Nf ZeNuaBoXAboWaoUIa10GDTf+FOSU245QKR4tCNFx8I3X6jrQDfMzSJYDEgcDuVBDU2JJ HZnHrEUNYhIldYfzKvaf09C9ruj8fQbjrlI55/DX7RUpedgKH44Oqa60HFK+dNyLuXGX DXnBI3uvKDqhiYPn+TV/eEAbIn/kn3nW1c9aBOJ5cthkEq244D4kqMAZyIctXtm35DBm pukw== X-Gm-Message-State: AOAM530+r5Dpb4vs0m9vdp2MgC1oHRiwu8j3bUcmClQm5ZEr1ECxB1R+ PRUzQIXvERufMqsXjMJr4ZURRw== X-Google-Smtp-Source: ABdhPJx77kifp8RGOMEluU2Yx0AvabGAllw9zyaPv/0lhbrR2NS6U+KBh4ugFKXFnmWOTjBVHzRObg== X-Received: by 2002:a1c:2358:: with SMTP id j85mr11972972wmj.1.1633549879474; Wed, 06 Oct 2021 12:51:19 -0700 (PDT) Received: from ?IPv6:2a01:e34:ed2f:f020:278:1f59:2992:87fe? ([2a01:e34:ed2f:f020:278:1f59:2992:87fe]) by smtp.googlemail.com with ESMTPSA id j11sm3746772wrp.27.2021.10.06.12.51.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 Oct 2021 12:51:18 -0700 (PDT) Subject: Re: [PATCH v2 0/2] Add a generic virtual thermal sensor To: "Rafael J. Wysocki" Cc: Alexandre Bailon , "Zhang, Rui" , Amit Kucheria , Linux PM , Linux Kernel Mailing List , ben.tseng@mediatek.com, Kevin Hilman , Matthias Kaehlcke References: <20210917072732.611140-1-abailon@baylibre.com> <7cddcdb7-4efd-bfdb-3d86-f5862ea0b7fe@baylibre.com> <8a9e5f13-6253-2d0d-35a8-789090af4521@linaro.org> <794e62ea-d867-3827-de5f-24ddc86c3524@linaro.org> <4446577e-c7fa-daeb-e0fe-8a530633ef5d@baylibre.com> <03aeb132-bc0c-93f7-c7db-8575a665d2a7@linaro.org> From: Daniel Lezcano Message-ID: <566c8f70-1445-caef-811d-398729869719@linaro.org> Date: Wed, 6 Oct 2021 21:51:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/10/2021 20:00, Rafael J. Wysocki wrote: > On Wed, Oct 6, 2021 at 6:06 PM Daniel Lezcano wrote: >> >> On 05/10/2021 18:45, Rafael J. Wysocki wrote: >>> On Mon, Oct 4, 2021 at 3:42 PM Daniel Lezcano wrote: >>>> >>>> On 04/10/2021 12:24, Alexandre Bailon wrote: >>>>> >>>>> On 9/22/21 10:10 AM, Daniel Lezcano wrote: >>>>>> On 20/09/2021 15:12, Alexandre Bailon wrote: >>>>>>> On 9/17/21 4:03 PM, Daniel Lezcano wrote: >>>>>>>> On 17/09/2021 15:33, Alexandre Bailon wrote: >>>>>>>>> Hi Daniel, >>>>>>>>> >>>>>>>>> On 9/17/21 2:41 PM, Daniel Lezcano wrote: >>>>>>>>>> On 17/09/2021 09:27, Alexandre Bailon wrote: >>>>>>>>>>> This series add a virtual thermal sensor. >>>>>>>>>>> It could be used to get a temperature using some thermal sensors. >>>>>>>>>>> Currently, the supported operations are max, min and avg. >>>>>>>>>>> The virtual sensor could be easily extended to support others >>>>>>>>>>> operations. >>>>>>>>>>> >>>>>>>>>>> Note: >>>>>>>>>>> Currently, thermal drivers must explicitly register their sensors to >>>>>>>>>>> make them >>>>>>>>>>> available to the virtual sensor. >>>>>>>>>>> This doesn't seem a good solution to me and I think it would be >>>>>>>>>>> preferable to >>>>>>>>>>> update the framework to register the list of each available sensors. >>>>>>>>>> Why must the drivers do that ? >>>>>>>>> Because there are no central place where thermal sensor are >>>>>>>>> registered. >>>>>>>>> The only other way I found was to update thermal_of.c, >>>>>>>>> to register the thermal sensors and make them available later to the >>>>>>>>> virtual thermal sensor. >>>>>>>>> >>>>>>>>> To work, the virtual thermal need to get the sensor_data the ops from >>>>>>>>> the thermal sensor. >>>>>>>>> And as far I know, this is only registered in thermal_of.c, in the >>>>>>>>> thermal zone data >>>>>>>>> but I can't access it directly from the virtual thermal sensor. >>>>>>>>> >>>>>>>>> How would you do it ? >>>>>>>> Via the phandles when registering the virtual sensor ? >>>>>>> As far I know, we can't get the ops or the sensor_data from the phandle >>>>>>> of a thermal sensor. >>>>>>> The closest solution I found so far would be to aggregate the thermal >>>>>>> zones instead of thermal sensors. >>>>>>> thermal_zone_device has the data needed and a thermal zone could be find >>>>>>> easily using its name. >>>>>> Yeah, the concept of the thermal zone and the sensor are very close. >>>>>> >>>>>> There is the function in thermal_core.h: >>>>>> >>>>>> -> for_each_thermal_zone() >>>>>> >>>>>> You should be able for each 'slave' sensor, do a lookup to find the >>>>>> corresponding thermal_zone_device_ops. >>>>>> >>>>>>> But, using a thermal_zone_device, I don't see how to handle module >>>>>>> unloading. >>>>>> I think try_module_get() / module_put() are adequate for this situation >>>>>> as it is done on an external module and we can not rely on the exported >>>>>> symbols. >>>>> I don't see how it would be possible to use these functions. >>>>> The thermal zone doesn't have the data required to use it. >>>> >>>> Actually I was able to crash the kernel by doing: >>>> >>>> console 1: >>>> >>>> while $(true); do insmod && rmmod ; done >>>> >>>> console 2: >>>> >>>> while $(true); cat /sys/class/thermal/thermal_zone0/temp; done >>>> >>>> So there is something wrong already in the thermal framework. >>> >>> Hmmm. >>> >>>> IMO, the first thing would be to fix this critical issue by getting the >>>> sensor module refcount when the thermal zone is enabled and dropping it >>>> when it is disabled. >>>> >>>> With that fixed, perhaps it will possible to get the device associated >>>> with the sensor and then try_module_get(dev->driver->owner) >>>> >>>>> Maybe a more easier way is to use the thermal_zone_device mutex. >>>>> If I get a lock before to use the thermal_zone_device ops, I have the >>>>> guaranty that module won't be unloaded. >>> >>> That would be my approach too. >> >> The mutex is private to the thermal core. The virtual sensor should not >> touch it :/ >> >> Perhaps, it can work with a private spin_lock with a try_spinlock() ? > > IIUC this is a case when module A refers to some memory in module B > and if the latter goes away, an access to that memory from the former > is a use-after-free, so it is not sufficient to use a local spinlock. > This can be avoided by having a lock and a flag such that the flag is > set under the lock by module B when making the memory in question > available and cleared under the lock when freeing that memory. Then, > module A needs to check the flag under the lock on every access to > that memory. Also, the lock and the flag must be accessible all the > time to both modules (ie. must not go away along with any of them if > they don't depend on each other). Just to clarify, the idea behind the virtual sensor is the same as the network bridge: the network interfaces are not aware they are attached to a bridge. The virtual sensor should attach and detach the physical sensors. The sensors should not un|register themselves from|to the virtual sensor, nor having specific code related to the virtual sensor. De facto, all the existing sensors will be compatible with the virtual sensor. Do we have a solution other than grabbing a module reference and self-contained to the virtual sensor ? >>>>> When a "thermal of sensor" is unloaded, it calls >>>>> thermal_zone_of_sensor_unregister which takes a lock before >>>>> update ops. >>>> >>>> I'm not sure to understand. The goal is to have the refcount on the >>>> modules to be incremented when the virtual sensor is using them. >>> >>> IMO the goal is to prevent the code from crashing when modules get >>> unloaded. I'm not really sure if refcounts alone are sufficient for >>> that. >> >> The problem is in the loop: >> >> +static int virtual_thermal_sensor_get_temp(void *data, int *temperature) >> +{ >> + struct virtual_thermal_sensor *sensor = data; >> + int max_temp = INT_MIN; >> + int temp; >> + int i; >> + >> + for (i = 0; i < sensor->count; i++) { >> + struct thermal_sensor_data *hw_sensor; >> + >> + hw_sensor = &sensor->sensors[i]; >> + if (!hw_sensor->ops) >> + return -ENODEV; >> + >> + hw_sensor->ops->get_temp(hw_sensor->sensor_data, &temp); >> + max_temp = sensor->aggr_temp(max_temp, temp); >> + } >> + >> + *temperature = max_temp; >> + >> + return 0; >> +} >> >> If one of the sensor is unloaded when get_temp is called, >> hw_sensor->ops->get_temp will crash. > > Right. > > Dereferencing hw_sensor itself is not safe in this loop if the module > holding the memory pointed to by it may go away. > > However, presumably, the hw_sensor object needs to be registered with > the core in order to be used here and unregistered when it goes away, > so it looks like this loop could use a wrapper like > thermal_get_sensor_temp(hw_sensor, &temp) which would return a > negative error code if the sensor in question went away. > > Of course, that would require the core to check the list of available > sensors every time, but that may be implemented efficiently with the > help of an xarray, for example. > >> So the proposal is virtual_sensor_add_sensor() does try_get_module() >> and virtual_sensor_remove_sensor() does put_module(). >> >> The ref on the 'slave' modules will be release only if the virtual >> sensor is unregistered. >> >> So until, the virtual sensor is unregistered, the 'slaves' modules can >> not be unloaded. >> >> That is what we find with eg. the wifi modules. > > Yes, but that's a bit cumbersome from the sysadmin perspective IMO, > especially when one wants to unload one of the modules and doesn't > know exactly what other modules hold references to it. Well, sysadmin are not supposed to unload a thermal sensor and I would say if he unloads the module, which is not an usual operation, up to him to 'lsmod' and check what module is holding the reference. > IIUC ref_module() might be nicer, but it is still more convenient if > the modules can just go away independently. "for desesperate users" :) -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog