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[75.142.250.213]) by smtp.gmail.com with ESMTPSA id 22sm5286695oie.54.2020.10.10.11.07.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 10 Oct 2020 11:07:37 -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> <20201010175751.GA5714@paulmck-ThinkPad-P72> From: Tom Rix Message-ID: Date: Sat, 10 Oct 2020 11:07:34 -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: <20201010175751.GA5714@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 10/10/20 10:57 AM, Paul E. McKenney wrote: > 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. ;-) Ok. I have another a change in other area queued up. I'll improve it's and future comments. Tom > 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; >> }