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 X-Spam-Level: X-Spam-Status: No, score=-7.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 54B9CC433E0 for ; Tue, 23 Jun 2020 15:43:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2700120780 for ; Tue, 23 Jun 2020 15:43:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="XHMK0nkJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733012AbgFWPnJ (ORCPT ); Tue, 23 Jun 2020 11:43:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37760 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732943AbgFWPnI (ORCPT ); Tue, 23 Jun 2020 11:43:08 -0400 Received: from mail-qt1-x842.google.com (mail-qt1-x842.google.com [IPv6:2607:f8b0:4864:20::842]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8F528C061755 for ; Tue, 23 Jun 2020 08:43:08 -0700 (PDT) Received: by mail-qt1-x842.google.com with SMTP id d27so15727315qtg.4 for ; Tue, 23 Jun 2020 08:43:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=7VlXbhyUYUJUN03OeBT3cNSBeivqJZy9NZ8rrKrnItc=; b=XHMK0nkJpJiB/mfYekI7NlotB7iw2rWvBlNASwjNXbiqVlJx9YPDxzUGpq0I6V0IVz 3dUh0t3iOxO3WerytqVG2IQRykKI+aLp6UuCtbiZNmDop7TQ3ieIIG8r3L8MLKLjuzRA SAfrE0MaJ8Gym3PJQ6b3U4meIer5hPzZHNolE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=7VlXbhyUYUJUN03OeBT3cNSBeivqJZy9NZ8rrKrnItc=; b=tX2LkSsCzI/X//TYlRsLOI2CSNX9Vh7K55nbocaatg/ZnXcI6yuGIjzojAqOq7qjmg 3z5vHwCfaaquUoqfyH15bc1eC8oxkE+UKaETLUVahPcdyrryXwpDO2tS7ZDFK9xLsx9q HnaxKjsSpC88fQYeSgRj27AnwGqm1oFONugnL1XKTdoDtdV64ziZEK2kcl4rJcQo7qkR K4JXMnPltE4lqqEsL7/RsUTYyVQbaJ+WBYU3cjB3PRtkBdNd4A4rLBki/iSWgD/DVB4V AgXIzkzMfPDX5zBfbTkyx/LBiFLaUp4OthTNOAY4nQ0mkLtYxqsmUh8cFH/bQrQk8JfB Pu2g== X-Gm-Message-State: AOAM532uy3cFs2eBbQVfTq63YLKVUTygIJBio2WtZu7q0riyf9H7AtUf 11fU8faHR5eNbpdFcs7v2ZUq7Q== X-Google-Smtp-Source: ABdhPJzS4wZ8vqkhsqbp8WTevzokYycPZg8FJKzmZA1Ni6+5zrRBC2SqmbadEF85GYSZtTl9BbqUEw== X-Received: by 2002:aed:221a:: with SMTP id n26mr15773016qtc.8.1592926987737; Tue, 23 Jun 2020 08:43:07 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id d140sm881921qkc.22.2020.06.23.08.43.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Jun 2020 08:43:07 -0700 (PDT) Date: Tue, 23 Jun 2020 11:43:06 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: Madhuparna Bhowmik , Paolo Bonzini , sean.j.christopherson@intel.com, vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com, tglx@linutronix.de, bp@alien8.de, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, frextrite@gmail.com, linux-kernel-mentees@lists.linuxfoundation.org Subject: Re: [PATCH v2] kvm: Fix false positive RCU usage warning Message-ID: <20200623154306.GF9005@google.com> References: <20200516082227.22194-1-madhuparnabhowmik10@gmail.com> <9fff3c6b-1978-c647-16f7-563a1cdf62ff@redhat.com> <20200623150236.GD9005@google.com> <20200623153036.GB9914@madhuparna-HP-Notebook> <20200623153901.GG9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200623153901.GG9247@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 23, 2020 at 08:39:01AM -0700, Paul E. McKenney wrote: > On Tue, Jun 23, 2020 at 09:00:36PM +0530, Madhuparna Bhowmik wrote: > > On Tue, Jun 23, 2020 at 11:02:36AM -0400, Joel Fernandes wrote: > > > On Tue, Jun 23, 2020 at 09:39:53AM +0200, Paolo Bonzini wrote: > > > > On 16/05/20 10:22, madhuparnabhowmik10@gmail.com wrote: > > > > > From: Madhuparna Bhowmik > > > > > > > > > > Fix the following false positive warnings: > > > > > > > > > > [ 9403.765413][T61744] ============================= > > > > > [ 9403.786541][T61744] WARNING: suspicious RCU usage > > > > > [ 9403.807865][T61744] 5.7.0-rc1-next-20200417 #4 Tainted: G L > > > > > [ 9403.838945][T61744] ----------------------------- > > > > > [ 9403.860099][T61744] arch/x86/kvm/mmu/page_track.c:257 RCU-list traversed in non-reader section!! > > > > > > > > > > and > > > > > > > > > > [ 9405.859252][T61751] ============================= > > > > > [ 9405.859258][T61751] WARNING: suspicious RCU usage > > > > > [ 9405.880867][T61755] ----------------------------- > > > > > [ 9405.911936][T61751] 5.7.0-rc1-next-20200417 #4 Tainted: G L > > > > > [ 9405.911942][T61751] ----------------------------- > > > > > [ 9405.911950][T61751] arch/x86/kvm/mmu/page_track.c:232 RCU-list traversed in non-reader section!! > > > > > > > > > > Since srcu read lock is held, these are false positive warnings. > > > > > Therefore, pass condition srcu_read_lock_held() to > > > > > list_for_each_entry_rcu(). > > > > > > > > > > Reported-by: kernel test robot > > > > > Signed-off-by: Madhuparna Bhowmik > > > > > --- > > > > > v2: > > > > > -Rebase v5.7-rc5 > > > > > > > > > > arch/x86/kvm/mmu/page_track.c | 6 ++++-- > > > > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c > > > > > index ddc1ec3bdacd..1ad79c7aa05b 100644 > > > > > --- a/arch/x86/kvm/mmu/page_track.c > > > > > +++ b/arch/x86/kvm/mmu/page_track.c > > > > > @@ -229,7 +229,8 @@ void kvm_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new, > > > > > return; > > > > > > > > > > idx = srcu_read_lock(&head->track_srcu); > > > > > - hlist_for_each_entry_rcu(n, &head->track_notifier_list, node) > > > > > + hlist_for_each_entry_rcu(n, &head->track_notifier_list, node, > > > > > + srcu_read_lock_held(&head->track_srcu)) > > > > > if (n->track_write) > > > > > n->track_write(vcpu, gpa, new, bytes, n); > > > > > srcu_read_unlock(&head->track_srcu, idx); > > > > > @@ -254,7 +255,8 @@ void kvm_page_track_flush_slot(struct kvm *kvm, struct kvm_memory_slot *slot) > > > > > return; > > > > > > > > > > idx = srcu_read_lock(&head->track_srcu); > > > > > - hlist_for_each_entry_rcu(n, &head->track_notifier_list, node) > > > > > + hlist_for_each_entry_rcu(n, &head->track_notifier_list, node, > > > > > + srcu_read_lock_held(&head->track_srcu)) > > > > > if (n->track_flush_slot) > > > > > n->track_flush_slot(kvm, slot, n); > > > > > srcu_read_unlock(&head->track_srcu, idx); > > > > > > > > > > > > > Hi, sorry for the delay in reviewing this patch. I would like to ask > > > > Paul about it. > > > > > > > > While you're correctly fixing a false positive, hlist_for_each_entry_rcu > > > > would have a false _negative_ if you called it under > > > > rcu_read_lock/unlock and the data structure was protected by SRCU. This > > > > is why for example srcu_dereference is used instead of > > > > rcu_dereference_check, and why srcu_dereference uses > > > > __rcu_dereference_check (with the two underscores) instead of > > > > rcu_dereference_check. Using rcu_dereference_check would add an "|| > > > > rcu_read_lock_held()" to the condition which is wrong. > > > > > > > > I think instead you should add hlist_for_each_srcu and > > > > hlist_for_each_entry_srcu macro to include/linux/rculist.h. > > > > > > > > There is no need for equivalents of hlist_for_each_entry_continue_rcu > > > > and hlist_for_each_entry_from_rcu, because they use rcu_dereference_raw. > > > > However, it's not documented why they do so. > > > > > > You are right, this patch is wrong, we need a new SRCU list macro to do the > > > right thing which would also get rid of the last list argument. > > > > > Can we really get rid of the last argument? We would need the > > srcu_struct right for checking? > > Agreed! However, the API could be simplified by passing in a pointer to > the srcu_struct instead of a lockdep expression. An optional lockdep > expression might still be helpful for calls from the update side, > of course. That's true! thanks, - Joel