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 040BAC4332F for ; Fri, 29 Apr 2022 13:38:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1376287AbiD2NlU (ORCPT ); Fri, 29 Apr 2022 09:41:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56168 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1376309AbiD2Nky (ORCPT ); Fri, 29 Apr 2022 09:40:54 -0400 Received: from bhuna.collabora.co.uk (bhuna.collabora.co.uk [46.235.227.227]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E1EF1CB02C; Fri, 29 Apr 2022 06:37:35 -0700 (PDT) Received: from [127.0.0.1] (localhost [127.0.0.1]) (Authenticated sender: kholk11) with ESMTPSA id 27DF61F46597 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1651239454; bh=kcEQjWB3+aoh/KNYmS+gc/t6zz5Iohch35uok/tdZ2g=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ebK2702xHh6jZr+XuacSG3T6K77O0e6jE4yrdtu4TG85TlKoWWfyhRSUooxkXDMXR 0OoI3Ng05ua+RkKFNWUxH4AKEsV2ra3A4nupIfjQHulZQbrn6NeJ3nCemO0i8EO37n Zy4XpKKUqXxOTV8OlHIP6DfbqvUFKinAR4IbQvSyfDWJSguh8+h9IFwVR4AczojijI fHPfWMfaBjbACP1E9gfPgSRultAXHSnBqumARBke0iTzk2dgTcyd8nlMhywjJYwjXY 90Lw1KN5skaMwO/WgwifwowJDIbGtdpeo4AnFoss0MfbjjpxdBGDAD+TB59o8vj/XP k4LE+kVxfrPkg== Message-ID: <5f252726-3757-b063-d34f-c76beaa5d31b@collabora.com> Date: Fri, 29 Apr 2022 15:37:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Subject: Re: [PATCH] rpmsg: mtk_rpmsg: Fix circular locking dependency Content-Language: en-US To: Mathieu Poirier Cc: bjorn.andersson@linaro.org, matthias.bgg@gmail.com, pihsun@chromium.org, linux-remoteproc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org, kernel@collabora.com References: <20220114144737.375621-1-angelogioacchino.delregno@collabora.com> <20220428173148.GA2268704@p14s> From: AngeloGioacchino Del Regno In-Reply-To: <20220428173148.GA2268704@p14s> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Il 28/04/22 19:31, Mathieu Poirier ha scritto: > On Fri, Jan 14, 2022 at 03:47:37PM +0100, AngeloGioacchino Del Regno wrote: >> During execution of the worker that's used to register rpmsg devices >> we are safely locking the channels mutex but, when creating a new >> endpoint for such devices, we are registering a IPI on the SCP, which >> then makes the SCP to trigger an interrupt, lock its own mutex and in >> turn register more subdevices. >> This creates a circular locking dependency situation, as the mtk_rpmsg >> channels_lock will then depend on the SCP IPI lock. >> >> [ 18.014514] Possible unsafe locking scenario: >> [ 18.014515] CPU0 CPU1 >> [ 18.014517] ---- ---- >> [ 18.045467] lock(&mtk_subdev->channels_lock); >> [ 18.045474] lock(&scp->ipi_desc[i].lock); >> [ 18.228399] lock(&mtk_subdev->channels_lock); >> [ 18.228405] lock(&scp->ipi_desc[i].lock); >> [ 18.264405] > > I finally understand the problem, something that would have been impossible > without the pastebin you provided in your latest email. Please add the content > of that pastebin to the changelog and send another revision (checkpatch > warnings can be ignored). > Thanks for giving it another look... I'll add a cover letter with the contents of the pastebin to avoid possible confusion for anyone looking at the patch. >> >> To solve this, simply unlock the channels_lock mutex before calling >> mtk_rpmsg_register_device() and relock it right after, as safety is >> still ensured by the locking mechanism that happens right after >> through SCP. > > The integrity of the subdev->channels list is guaranteed by relocking the > mutex, I'm not sure what "through SCP" adds to the sentence. I'll clarify the commit message. > >> Notably, mtk_rpmsg_register_device() does not even require locking. >> > > I don't agree with the above sentence - if locking doesn't happen in > mtk_rpmsg_create_device(), there can be two CPUs accessing the list at the same > time. > That's right, I have largely underestimated that for some reason, sorry about that. Regards, Angelo > Thanks, > Mathieu >