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[75.142.250.213]) by smtp.gmail.com with ESMTPSA id b23sm3236942qkh.68.2020.10.09.21.47.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Oct 2020 21:47:26 -0700 (PDT) Subject: Re: [PATCH] rcutorture: remove unneeded check To: paulmck@kernel.org 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 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> From: Tom Rix Message-ID: <945d3540-0d22-4e7f-d819-1b5c4e836354@redhat.com> Date: Fri, 9 Oct 2020 21:47:20 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20201010025743.GN29330@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/9/20 7:57 PM, 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. >>>>> >>>>> 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? Looks fine. Thanks Tom > > 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; > } >