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

["Björn Töpel" <bjorn@kernel.org>]

Guo Ren <guoren@kernel.org> writes:

> On Mon, Jan 30, 2023 at 11:49 PM Mark Rutland <mark.rutland@arm.com> wrote:
>>
>> Hi Bjorn,
>>
>> On Mon, Jan 30, 2023 at 04:28:15PM +0100, Björn Töpel wrote:
>> > Guo Ren <guoren@kernel.org> 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@chromium.org/

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