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 7D392C04A68 for ; Thu, 28 Jul 2022 10:50:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236345AbiG1KuZ (ORCPT ); Thu, 28 Jul 2022 06:50:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48868 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236532AbiG1KuK (ORCPT ); Thu, 28 Jul 2022 06:50:10 -0400 Received: from mx0b-001ae601.pphosted.com (mx0b-001ae601.pphosted.com [67.231.152.168]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 927865A2EA for ; Thu, 28 Jul 2022 03:50:04 -0700 (PDT) Received: from pps.filterd (m0077474.ppops.net [127.0.0.1]) by mx0b-001ae601.pphosted.com (8.17.1.5/8.17.1.5) with ESMTP id 26S84P8O013301; Thu, 28 Jul 2022 05:49:59 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cirrus.com; h=message-id : date : mime-version : subject : to : cc : references : from : in-reply-to : content-type : content-transfer-encoding; s=PODMain02222019; bh=DRVfCCRNsnENGtq2cW3SlRNLo38VF3FslJXOnUBH2UI=; b=O3/dbz7qB27x+Zq9tOBxX5vHJ6WO2OynJc6HdqSDPj282EG1rRyx523RxjJExlk8fhWY RVYDG/HxmFU0NByOqX3EHJmI79rU6OmPMZSCKDx+XcQpRr5uzpAWzIxWAaZrxXfhyHmA WBBTi//v2uGinHVxf4jWy0v/ksD3UFySA8iKhjC+UyWOfWD+WgFef4rkyTEiRNBPcTuf Hm2jSxt9SV1BCVgvSzyZ5lnZojUvVF67NlqZMPqRU4BVw8TAHns6kksMVaY6nWbutZd4 DcorerWFgWvq84c57GTjfOkiNimS23qEVZcgVd16drzFXLkItZHlvEw79bfkHiF4mXid dQ== Received: from ediex02.ad.cirrus.com ([84.19.233.68]) by mx0b-001ae601.pphosted.com (PPS) with ESMTPS id 3hgddp6dba-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 28 Jul 2022 05:49:59 -0500 Received: from ediex01.ad.cirrus.com (198.61.84.80) by ediex02.ad.cirrus.com (198.61.84.81) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1118.9; Thu, 28 Jul 2022 05:49:58 -0500 Received: from ediswmail.ad.cirrus.com (198.61.86.93) by ediex01.ad.cirrus.com (198.61.84.80) with Microsoft SMTP Server id 15.2.1118.9 via Frontend Transport; Thu, 28 Jul 2022 05:49:58 -0500 Received: from [198.90.251.95] (edi-sw-dsktp-006.ad.cirrus.com [198.90.251.95]) by ediswmail.ad.cirrus.com (Postfix) with ESMTP id 266D72C5; Thu, 28 Jul 2022 10:49:58 +0000 (UTC) Message-ID: <85bb9c36-33e9-c3e2-1838-9ef45311669b@opensource.cirrus.com> Date: Thu, 28 Jul 2022 11:49:57 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH] component: try_module_get() to prevent unloading while in use Content-Language: en-US To: Greg KH CC: , , , References: <20220725160859.1274472-1-rf@opensource.cirrus.com> <4165774b-2b96-83d1-67eb-f7c49dd8041e@opensource.cirrus.com> From: Richard Fitzgerald In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Proofpoint-GUID: naVEaElw3sYTSuWtQs-l5VtxEAuGjdDA X-Proofpoint-ORIG-GUID: naVEaElw3sYTSuWtQs-l5VtxEAuGjdDA X-Proofpoint-Spam-Reason: safe Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26/07/2022 18:28, Greg KH wrote: > On Tue, Jul 26, 2022 at 11:32:28AM +0100, Richard Fitzgerald wrote: >> On 25/07/2022 19:09, Greg KH wrote: >>> On Mon, Jul 25, 2022 at 05:08:59PM +0100, Richard Fitzgerald wrote: >>>> Call try_module_get() on a component before attempting to call its >>>> bind() function, to ensure that a loadable module cannot be >>>> unloaded while we are executing its bind(). >>> >>> How can bind be called while the module is unloaded? >>> >> >> I didn't say it could. What I said is "unloaded while we are executing >> its bind()". Maybe that's already guaranteed to be safe somehow. It's >> actually the problem below that I was trying to fix but placing the >> try_module_get() before the bind() rather than after bind() seemed a >> trivial extra safety. > > It should be safe, bind() can't race with module remove as the driver > core locks will handle this. > >>>> If the bind is successful the module_put() is called only after it >>>> has been unbound. This ensures that the module cannot be unloaded >>>> while it is in use as an aggregate device. >>> >>> That's almost never the correct thing to do, what problem is this >>> solving? >>> >> >> What I see is that when a loadable module has been made part of an >> aggregate it is still possible to rmmod'd it. >> >> An alternative workaround would be for the parent to softdep to every >> driver that _might_ provide the aggregated components. Softdeps aren't >> unusual (we use it in some drivers that are directly related but don't >> directly link into each other). But to me this feels like a hack when >> used with the component framework - isn't the idea that the parent >> doesn't know (or doesn't need to know) which drivers will be aggregated? >> Wouldn't it be better that when a component driver is bound into an >> aggregate its module is automatically marked in-use? >> >> If there's a better way to mark the module in-use while is it bound >> into an aggregate, let me know and I'll look at implementing it. > > No module references should be incremented if a device is bound to a > driver, that's the old (1990's) way of thinking. If a module wants to > be unloaded, let it, and clean up everything that it was > controlling/talking to before the module remove is finished. > > That's the way all busses should be working, you don't increment a > module count when a driver binds to a device, otherwise how would you > unload a module that was being used at all? > > So just remove the components controlled by the module properly when it > is removed and all should be good. > > Do you have example code in the kernel tree today that does not properly > do this? Why not just fix that instead? > The actual code I'm working on isn't upstream yet, but it's a derivative of the way these two interoperate: sound/pci/hda/patch_realtek sound/pci/hda/cs35l41_hda.c In these systems the host audio interface is HDA but the amps are not HDA devices. Audio goes through the Realtek HDA codec to the amp but the amp is on a different bus (i2c or spi). The modules in the HDA stack all get marked as in-use, but the amp driver doesn't. So if it's unloaded the audio system is left in a limbo state where ALSA and the HDA stack are still up but the amp driver code has gone. However I realised that my try_module_get() isn't a fix anyway. It's claiming use of the module implementing the component but not of the bus that owns that module. I assume that's what you were referring to by having to deal with an unload instead of trying to prevent the unload. (And yes, I'm aware that in that patch_realtek.c it's missing locking around the shared struct to prevent it being accessed during a bind and unbind.) > thanks, > > greg k-h