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=-8.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, 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 59D88C433E7 for ; Fri, 9 Oct 2020 23:55:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 13B91221E9 for ; Fri, 9 Oct 2020 23:55:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602287712; bh=AQvna38hRSTE9+h7oWP0R7rRfzl0SoNSmK5pNJWsFxY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=DThuVZhcgW1+qK2UP9sWrkEU/heAbvtMtTmylN9xWjGFQZuHylTiV/fJtsfdA5B7O ft04N1Mrvhx61jWtsvsuY2wCBxnSGhnOOxU09QDQpp8HRkdANJIvCa3qCpD5o84/+/ 7UxrqvfSt5TZI6Vu7c/nBDMMaXem/3ZL9IMi0RtE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728487AbgJIXyc (ORCPT ); Fri, 9 Oct 2020 19:54:32 -0400 Received: from mail.kernel.org ([198.145.29.99]:41126 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728275AbgJIXux (ORCPT ); Fri, 9 Oct 2020 19:50:53 -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 C307C208C7; Fri, 9 Oct 2020 23:50:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602287433; bh=AQvna38hRSTE9+h7oWP0R7rRfzl0SoNSmK5pNJWsFxY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=LXKdzvvVAgmfdVDRctlhG/Lbcq6SnlKp1fm7tw0h/+iyodCtzGxeTbnqLh9M5WqGU vUMuJzRWFoe39r1C6guFe3BhUJkoN7eSCX4UWdjLKj+hywrNXT2Bdj7bWHTG0v50ns Px2vi6yRGwNGKnjTkkvWMgpHjGmgEHNs2RboiUpg= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 7D57B3522837; Fri, 9 Oct 2020 16:50:33 -0700 (PDT) Date: Fri, 9 Oct 2020 16:50:33 -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: <20201009235033.GM29330@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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <03e3eeed-6072-ccb8-a9c6-c79a99c701b8@redhat.com> 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 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. > > > > 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? Thanx, Paul ------------------------------------------------------------------------ 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; }