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=-11.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 4CC1CC35266 for ; Sat, 10 Oct 2020 22:51:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1837A221FE for ; Sat, 10 Oct 2020 22:51:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602370318; bh=FERYGj39R6l6EZJxDuK7UmyVwM31uASPWiVprTEV/1E=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=Yk93Zo+KrAKQPBvAniJtWlM+jeKf86rJaFWVKSQshhbWjwcvnZNUU+4YJ56Crt3TZ Xs/nCv8QvGoSNm5amUNJJ5O0J6X6p05z4dZWOkZJT221Oss6Xtn+IVsUK+pgcYxSDD p7RUV2D9ekSB9odgrm3NzJSA3aZIRifFqad4C09g= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733190AbgJJWob (ORCPT ); Sat, 10 Oct 2020 18:44:31 -0400 Received: from mail.kernel.org ([198.145.29.99]:48684 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731113AbgJJTCX (ORCPT ); Sat, 10 Oct 2020 15:02:23 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-104-11.bvtn.or.frontiernet.net [50.39.104.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C0406224B0; Sat, 10 Oct 2020 17:57:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602352671; bh=FERYGj39R6l6EZJxDuK7UmyVwM31uASPWiVprTEV/1E=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Ij3EzWpW2WmqIyWGcdwZSPxLiJXUejt0V7TCmuPLyunAXWXR47YptR0VT/F5Bm8l4 F3NObqqfh0zWfT2O1l6S2g61nz4+7xgOlnPhbxDUDM8h0/VV/YEuiNDkOMAGl8Yekc b9umrY1oKZJjgDikOtwmJT2Hth7c43MEI7wqrc8c= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 860C4352261C; Sat, 10 Oct 2020 10:57:51 -0700 (PDT) Date: Sat, 10 Oct 2020 10:57:51 -0700 From: "Paul E. McKenney" To: Tom Rix Cc: dave@stgolabs.net, josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, natechancellor@gmail.com, ndesaulniers@google.com, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, clang-built-linux@googlegroups.com Subject: Re: [PATCH] rcutorture: remove unneeded check Message-ID: <20201010175751.GA5714@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201009194736.2364-1-trix@redhat.com> <20201009201825.GL29330@paulmck-ThinkPad-P72> <03e3eeed-6072-ccb8-a9c6-c79a99c701b8@redhat.com> <20201009235033.GM29330@paulmck-ThinkPad-P72> <92f82632-adbd-ca85-d516-6540a49f01ab@redhat.com> <20201010025743.GN29330@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201010025743.GN29330@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 09, 2020 at 07:57:43PM -0700, Paul E. McKenney wrote: > On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote: > > > > On 10/9/20 4:50 PM, Paul E. McKenney wrote: > > > On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote: > > >> On 10/9/20 1:18 PM, Paul E. McKenney wrote: > > >>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote: > > >>>> From: Tom Rix > > >>>> > > >>>> clang static analysis reports this problem: > > >>>> > > >>>> rcutorture.c:1999:2: warning: Called function pointer > > >>>> is null (null dereference) > > >>>> cur_ops->sync(); /* Later readers see above write. */ > > >>>> ^~~~~~~~~~~~~~~ > > >>>> > > >>>> This is a false positive triggered by an earlier, later ignored > > >>>> NULL check of sync() op. By inspection of the rcu_torture_ops, > > >>>> the sync() op is never uninitialized. So this earlier check is > > >>>> not needed. > > >>> You lost me on this one. This check is at the very beginning of > > >>> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an > > >>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so, > > >>> where exactly is this check? > > >>> > > >>> In any case, the check is needed because all three functions are invoked > > >>> if there is a self-propagating RCU callback that ensures that there is > > >>> always an RCU grace period outstanding. > > >>> > > >>> Ah. Is clang doing local analysis and assuming that because there was > > >>> a NULL check earlier, then the pointer might be NULL later? That does > > >>> not seem to me to be a sound check. In this case, the diagnostic was clearly pointing out a latent bug, so my bad. So more of a code-review comment than a diagnostic. Therefore, if clang really wants to be in the code-review space, I suggest that it more clearly explain its code-review feedback. ;-) Thanx, Paul > > >>> So please let me know exactly what is causing clang to emit this > > >>> diagnostic. It might or might not be worth fixing this, but either way > > >>> I need to understand the situation so as to be able to understand the > > >>> set of feasible fixes. > > >>> > > >>> Thanx, Paul > > >> In rcu_prog_nr() there is check for for sync. > > >> > > >> if ( ... cur_op->sync ... > > >> > > >>    do something > > >> > > >> This flags in clang's static analyzer as 'could be null' > > >> > > >> later in the function, in a reachable block it is used > > >> > > >> cur_ops->sync() > > >> > > >> I agree this is not a good check that's why i said is was a false positive. > > >> > > >> However when looking closer at how cur_ops is set, it is never uninitialized. > > >> > > >> So the check is not needed. > > > OK, got it, and thank you for the explanation. > > > > > >> This is not a fix, the code works fine.  It is a small optimization. > > > Well, there really is a bug. Yes, right now all ->sync pointers are > > > non-NULL, but they have not been in the past and might not be in the > > > future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should > > > not be called or it should return immediately without doing anything. > > > > > > My current thought is something like the (untested) patch below, of > > > course with your Reported-by. > > > > > > Thoughts? > > > > Yes that would be fine. > > > > In in review these other cases need to be been take care of. > > I am having a difficult time interpreting this sentence, but will > optimistically assume that it means that you are good with this approach. > If my optimism is unwarranted, please let me know so I can fix whatever > might be broken. > > > Reported-by: Tom Rix > > How does the commit below look? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd > Author: Paul E. McKenney > Date: Fri Oct 9 19:51:55 2020 -0700 > > rcutorture: Don't do need_resched() testing if ->sync is NULL > > If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless > attempt to call through it. This commit therefore flags cases where > neither need_resched() nor call_rcu() forward-progress testing > can be performed due to NULL function pointers, and also causes > rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync() > is NULL. > > Reported-by: Tom Rix > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c > index beba9e7..44749be 100644 > --- a/kernel/rcu/rcutorture.c > +++ b/kernel/rcu/rcutorture.c > @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp, > unsigned long stopat; > static DEFINE_TORTURE_RANDOM(trs); > > - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) { > + if (!cur_ops->sync) > + return; // Cannot do need_resched() forward progress testing without ->sync. > + if (cur_ops->call && cur_ops->cb_barrier) { > init_rcu_head_on_stack(&fcs.rh); > selfpropcb = true; > } > @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void) > > if (!fwd_progress) > return 0; /* Not requested, so don't do it. */ > - if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || > - cur_ops == &rcu_busted_ops) { > + if ((!cur_ops->sync && !cur_ops->call) || > + !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) { > VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test"); > return 0; > }