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
> Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
> ---
> kernel/rcu/rcutorture.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7963c8..6efc03a1d623 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,7 @@ 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->call && cur_ops->cb_barrier) {
> init_rcu_head_on_stack(&fcs.rh);
> selfpropcb = true;
> }
> --
> 2.18.1
>
