Messages in this thread | | | Date | Tue, 08 May 2018 15:41:11 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features |
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Masami Hiramatsu wrote: > On Mon, 07 May 2018 13:41:53 +0530 > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote: >> >> >> >> I didn't understand that. Which code are you planning to remove? Can you >> >> please elaborate? I thought we still need to disable preemption in the >> >> ftrace handler. >> > >> > Yes, kprobe_ftrace_handler itself must be run under preempt disabled >> > because it depends on a per-cpu variable. What I will remove is the >> > redundant preempt disable/enable_noresched (unbalanced) pair in the >> > kprobe_ftrace_handler, and jprobe x86 ports which is no more used. >> >> Won't that break out-of-tree users depending on returning a non-zero >> value to handle preemption differently? You seem to have alluded to it >> earlier in the mail chain above where you said that this is not just for >> jprobes (though it was added for jprobes as the main use case). > > No, all users are in tree already (function override for bpf and error-injection).
Ok, so BPF error injection is a new user that can return a non-zero value from the pre handler. It looks like it can use KPROBES_ON_FTRACE too.
In that case, on function entry, we call into kprobe_ftrace_handler() which will call fei_kprobe_handler(), which can re-enable premption before returning 1. So, if you remove the additional prempt_disable()/enable_no_resched() in kprobe_ftrace_handler(), then it will become imbalanced, right?
> And also, for changing execution path by using kprobes, user handler must call > not only preempt_enable(), but also clear current_kprobe per-cpu variable which > is not exported to kmodules.
Ok, good point. And that means we don't have any external users any more.
Thanks, Naveen
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