Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity

[Guo Ren]

On Tue, Jan 31, 2023 at 3:12 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> Guo Ren <guoren@xxxxxxxxxx> writes:
>
> > On Mon, Jan 30, 2023 at 11:49 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >>
> >> Hi Bjorn,
> >>
> >> On Mon, Jan 30, 2023 at 04:28:15PM +0100, Björn Töpel wrote:
> >> > Guo Ren <guoren@xxxxxxxxxx> writes:
> >> >
> >> > >> In the serie of RISCV OPTPROBES [1], it patches a long-jump instructions pair
> >> > >> AUIPC/JALR in kernel text, so in order to ensure other CPUs does not execute
> >> > >> in the instructions that will be modified, it is still need to stop other CPUs
> >> > >> via patch_text API, or you have any better solution to achieve the purpose?
> >> > >  - The stop_machine is an expensive way all architectures should
> >> > > avoid, and you could keep that in your OPTPROBES implementation files
> >> > > with static functions.
> >> > >  - The stop_machine couldn't work with PREEMPTION, so your
> >> > > implementation needs to work with !PREEMPTION.
> >> >
> >> > ...and stop_machine() with !PREEMPTION is broken as well, when you're
> >> > replacing multiple instructions (see Mark's post at [1]). The
> >> > stop_machine() dance might work when you're replacing *one* instruction,
> >> > not multiple as in the RISC-V case. I'll expand on this in a comment in
> >> > the OPTPROBES v6 series.
> >>
> >> Just to clarify, my comments in [1] were assuming that stop_machine() was not
> >> used, in which case there is a problem with or without PREEMPTION.
> >>
> >> I believe that when using stop_machine(), the !PREEMPTION case is fine, since
> >> stop_machine() schedules work rather than running work in IRQ context on the
> >> back of an IPI, so no CPUs should be mid-sequnce during the patching, and it's
> >> not possible for there to be threads which are preempted mid-sequence.
> >>
> >> That all said, IIUC optprobes is going to disappear once fprobe is ready
> >> everywhere, so that might be moot.
> > The optprobes could be in the middle of a function, but fprobe must be
> > the entry of a function, right?
> >
> > Does your fprobe here mean: ?
> >
> > The Linux kernel configuration item CONFIG_FPROBE:
> >
> > prompt: Kernel Function Probe (fprobe)
> > type: bool
> > depends on: ( CONFIG_FUNCTION_TRACER ) && (
> > CONFIG_DYNAMIC_FTRACE_WITH_REGS ) && ( CONFIG_HAVE_RETHOOK )
> > defined in kernel/trace/Kconfig
>
> See the cover of [1]. It's about direct calls for BPF tracing (and more)
> on Arm, and you're completly right, that it's *not* related to optprobes
> at all.
>
> [1] https://lore.kernel.org/all/20221108220651.24492-1-revest@xxxxxxxxxxxx/
Thx for sharing :)

-- 
Best Regards
 Guo Ren