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@xxxxxxxxxx wrote: > >>>> From: Tom Rix <trix@xxxxxxxxxx> > >>>> > >>>> 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 <trix@xxxxxxxxxx> How does the commit below look? Thanx, Paul ------------------------------------------------------------------------ commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd Author: Paul E. McKenney <paulmck@xxxxxxxxxx> 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 <trix@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> 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; }