On Sun, 21 May 2023 22:26:37 +0200
Jiri Olsa [off-list ref] wrote:

On Sun, May 21, 2023 at 11:10:16PM +0800, Ze Gao wrote:

quoted

quoted

kprobe_multi/fprobe share the same set of attachments with fentry.
Currently, fentry does not filter with !rcu_is_watching, maybe
because this is an extreme corner case. Not sure whether it is
worthwhile or not.

Agreed, it's rare, especially after Peter's patches which push narrow
down rcu eqs regions
in the idle path and reduce the chance of any traceable functions
happening in between.

However, from RCU's perspective, we ought to check if rcu_is_watching
theoretically
when there's a chance our code will run in the idle path and also we
need rcu to be alive,
And also we cannot simply make assumptions for any future changes in
the idle path.
You know, just like what was hit in the thread.

quoted

Maybe if you can give a concrete example (e.g., attachment point)
with current code base to show what the issue you encountered and
it will make it easier to judge whether adding !rcu_is_watching()
is necessary or not.

I can reproduce likely warnings on v6.1.18 where arch_cpu_idle is
traceable but not on the latest version
so far. But as I state above, in theory we need it. So here is a
gentle ping :) .

hum, this change [1] added rcu_is_watching check to ftrace_test_recursion_trylock,
which we use in fprobe_handler and is coming to fprobe_exit_handler in [2]

I might be missing something, but it seems like we don't need another
rcu_is_watching call on kprobe_multi level

Good point! OK, then it seems we don't need it. The rethook continues to
use the rcu_is_watching() because it is also used from kprobes, but the
kprobe_multi doesn't need it.

Thank you,

jirka


[1] d099dbfd3306 cpuidle: tracing: Warn about !rcu_is_watching()
[2] https://lore.kernel.org/bpf/20230517034510.15639-4-zegao@tencent.com/ (local)

-- 
Masami Hiramatsu (Google) [off-list ref]