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=unavailable 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 7324CC433DF for ; Tue, 23 Jun 2020 15:02:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 411D220723 for ; Tue, 23 Jun 2020 15:02:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="E9w4bJSa" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732973AbgFWPCk (ORCPT ); Tue, 23 Jun 2020 11:02:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59644 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732801AbgFWPCj (ORCPT ); Tue, 23 Jun 2020 11:02:39 -0400 Received: from mail-qk1-x741.google.com (mail-qk1-x741.google.com [IPv6:2607:f8b0:4864:20::741]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B30CAC061755 for ; Tue, 23 Jun 2020 08:02:38 -0700 (PDT) Received: by mail-qk1-x741.google.com with SMTP id q198so11229039qka.2 for ; Tue, 23 Jun 2020 08:02:38 -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=1z1ehSCwot5VnE3TNEBjXXOGfLvJr+9MkOksrysPCMc=; b=E9w4bJSaqWf/pKpisHHt1S+BthSyfa0e5/L1Sq+lQv8OUIH0PphxurlrzMDvFKQ7iF 3KTnDZDNNgWGryp4CFe6qW97zQXbedEQrAo8NBVUe4qsOTKQvQUC2xRgeBfIF3aOWQbl itGNugF83gHq5dlbL/dzymckUmqzbu1BXAM68= 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=1z1ehSCwot5VnE3TNEBjXXOGfLvJr+9MkOksrysPCMc=; b=fR8ZIRIYEkz3ROiY/ZXi+kIsuYAzFh1SuavC+ittrHMLzPe0cSDAZ9JOzLtQ91wJ40 dqhjaZYCgRvtxrhNiICUJJDjeT3TRDL9UJKHU36tQJqzzJQR5kx+7r0teANaIXk696ls xVZLCPAQJPtm6PSX0Unz8u+fTWLHAzXWSUXP62lnnle44iGDIOzSOYO9CRBKLZGEQC/x 7zZP426Nn6mX2xgB+ABs8woSE6uh4jwb3/oawhdczwol8oENhjfR+TJXq5n3KLuswEHN 6pqPiEO+0aY4CHQr0KwFAxndN8AXfDwNJLGQ8Jpjq0gawylgZK88MU31sw1xR9mInjON uV5g== X-Gm-Message-State: AOAM530L69d7g0NZNAZlaeZS92qcejZOTOOow1WF2pxf7ILN4C/9rrgm Wy6X6qOiF1C8EWiizt8N6HEalpGeXXs= X-Google-Smtp-Source: ABdhPJyTSkcTrA3rEAn3O/WxA9L/aDtqy8cJk0ZXY9Bz0OBYsb6e1d9hSYWAguNsZHEfozmInbFSDw== X-Received: by 2002:a05:620a:b1a:: with SMTP id t26mr21010205qkg.473.1592924557749; Tue, 23 Jun 2020 08:02:37 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id d186sm737579qkb.110.2020.06.23.08.02.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Jun 2020 08:02:36 -0700 (PDT) Date: Tue, 23 Jun 2020 11:02:36 -0400 From: Joel Fernandes To: Paolo Bonzini Cc: madhuparnabhowmik10@gmail.com, 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, paulmck@kernel.org, frextrite@gmail.com, linux-kernel-mentees@lists.linuxfoundation.org, Paul McKenney Subject: Re: [PATCH v2] kvm: Fix false positive RCU usage warning Message-ID: <20200623150236.GD9005@google.com> References: <20200516082227.22194-1-madhuparnabhowmik10@gmail.com> <9fff3c6b-1978-c647-16f7-563a1cdf62ff@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9fff3c6b-1978-c647-16f7-563a1cdf62ff@redhat.com> 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 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.