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 3E8D4C433F5 for ; Sat, 19 Feb 2022 00:31:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240432AbiBSAbT (ORCPT ); Fri, 18 Feb 2022 19:31:19 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:43342 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240424AbiBSAbS (ORCPT ); Fri, 18 Feb 2022 19:31:18 -0500 Received: from mail-pl1-x62d.google.com (mail-pl1-x62d.google.com [IPv6:2607:f8b0:4864:20::62d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D3F1A12E9E7 for ; Fri, 18 Feb 2022 16:31:00 -0800 (PST) Received: by mail-pl1-x62d.google.com with SMTP id x11so8374489pll.10 for ; Fri, 18 Feb 2022 16:31:00 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=8aZ16UFG/qN8SDATqq+xbNjeyEK8F2U4AYeI8Q7W9Mw=; b=HArcINaTCCHq36tGrqoEk7mWbIGxWYrZtNbvjNwmDNrmBHrKF3eJlEmgsdsTdOVnMq gwU0FIiZ3lb3nTu1nqY4vVwwJnHUzAC66FxLehc7wPjjnFMa8nrC2vIKci/NI+cgdx0O 5mv4Zjt08mwVjFMhablIrVrKKZ0tJ7ZkS+DWkG6oHr8ISvrHErku953L0IAfmJ5H1IwE tpcPZ0e+eZl3ta4rPpfIMbwMD/rATwJYXVln33HESHVem5AggYKENrLDu9hCiM6AeD+L 65Y6x9O5zH7k0GE7uQdIZ4ZIvC3mSThjkE1YaZhLPfx0/4C+JHfEqBOdh+HVxKvriHUS 0uSA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=8aZ16UFG/qN8SDATqq+xbNjeyEK8F2U4AYeI8Q7W9Mw=; b=aCts3Bej83iIkP5tJlahWGkwnRidx3keDB2USbaXIvJox9I4DH0Hz9gd63ufTWY59X gh6qNO9R6f+yw2rrISxA/En8qH2BnCo8ZEXSVM9vufGmZQZApiSbA4f7oil7DqWmItKK P0Am5EUZj4eAWMGgndWrioFxSGZuEJSY16zNOeXbPN8++l6vY1jN6mtA4p13+uYVU3Kh wDrd/rxz/GSqxeMN+ozRK2z/jvDtXxAFIqCaAII0sjbFWc7h1UbcY5Cve8mbK3txfOHo hEcTl8KqC4l/SGpmgkczBGciVqRDosGUc3wSKFaFIGNOsOtMk5WhCN8kGcPK11KG09xL DQbA== X-Gm-Message-State: AOAM533f00qVh7NMW5kMWLkOzcLC5i9mQ/tIVGbpZehv9blBYnGp6QFX JQveGPI+cRuOYzgdqJPkW3mGMg== X-Google-Smtp-Source: ABdhPJzSAZ6U4sxN5IKhc/gevNuxGBEtDf2fuX3iCreS0CIEogpHitT+hzBHDWMZx2JytIHaobdHSg== X-Received: by 2002:a17:90b:e89:b0:1b9:19d0:432c with SMTP id fv9-20020a17090b0e8900b001b919d0432cmr14919792pjb.154.1645230660174; Fri, 18 Feb 2022 16:31:00 -0800 (PST) Received: from google.com (157.214.185.35.bc.googleusercontent.com. [35.185.214.157]) by smtp.gmail.com with ESMTPSA id x6sm4127791pfo.152.2022.02.18.16.30.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 18 Feb 2022 16:30:59 -0800 (PST) Date: Sat, 19 Feb 2022 00:30:56 +0000 From: Sean Christopherson To: Paolo Bonzini Cc: kernel test robot , Vipin Sharma , kbuild-all@lists.01.org, mkoutny@suse.com, tj@kernel.org, lizefan.x@bytedance.com, hannes@cmpxchg.org, dmatlack@google.com, jiangshanlai@gmail.com, kvm@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] KVM: Move VM's worker kthreads back to the original cgroup before exiting. Message-ID: References: <20220217061616.3303271-1-vipinsh@google.com> <202202172046.GuW8pHQc-lkp@intel.com> <3113f00a-e910-2dfb-479f-268566445630@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3113f00a-e910-2dfb-479f-268566445630@redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Thu, Feb 17, 2022, Paolo Bonzini wrote: > On 2/17/22 13:34, kernel test robot wrote: > > > 5859 reattach_err = cgroup_attach_task_all(current->real_parent, current); > > This needs to be within rcu_dereference(). As Vipin found out the hard way, cgroup_attach_task_all() can't be inside an RCU critical section as it might sleep due to mutex_lock(). My sched knowledge is sketchy at best, but I believe KVM just needs to guarantee the liveliness of real_parent when passing it to cgroup_attach_task_all(). Presumably kthreadd_task can (in theory) be killed/exiting while this is going on, but at that point I doubt anyone cares much about cgroup accuracy. So I think this can be: rcu_read_lock(); parent = rcu_dereference(current->real_parent); get_task_struct(parent); rcu_read_unlock(); (void)cgroup_attach_task_all(parent, current); put_task_struct(parent); > > 5860 if (reattach_err) { > > 5861 kvm_err("%s: cgroup_attach_task_all failed on reattach with err %d\n", > > 5862 __func__, reattach_err); Eh, I wouldn't bother logging the error, userspace can't take action on it, and if the system is OOM it's just more noise in the log to dig through.