Thanks Steven, I think we've come to a consensus on this.

The question here is whether bpf tracing fentry i.e.,
__bpf_prog_enter{_sleepable}
needs to check rcu_is_watching as well before using rcu related
calls. And Yonghong suggested making a change when there is
indeed some bad case occurring since it's rare the tracee is in the idle path.


Regards,
Ze

On Tue, May 23, 2023 at 10:10 PM Steven Rostedt [off-list ref] wrote:

[ Added a subject, as I always want to delete these emails as spam! ]

On Mon, 22 May 2023 10:07:42 +0800
Ze Gao [off-list ref] wrote:

quoted

Oops, I missed that. Thanks for pointing that out, which I thought is
conditional use of rcu_is_watching before.

One last point, I think we should double check on this
     "fentry does not filter with !rcu_is_watching"
as quoted from Yonghong and argue whether it needs
the same check for fentry as well.

Note that trace_test_and_set_recursion() (which is used by
ftrace_test_recursion_trylock()) checks for rcu_is_watching() and
returns false if it isn't (and the trylock will fail).

-- Steve