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SubjectRe: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features
Masami Hiramatsu wrote:
> On Fri, 4 May 2018 12:06:42 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
>> On Sat, 5 May 2018 00:48:28 +0900
>> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>>
>> > > Also, when looking at the kprobe code, I was looking at this
>> > > function:
>> > >
>> > > > /* Ftrace callback handler for kprobes -- called under preepmt disabed */
>> > > > void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>> > > > struct ftrace_ops *ops, struct pt_regs *regs)
>> > > > {
>> > > > struct kprobe *p;
>> > > > struct kprobe_ctlblk *kcb;
>> > > >
>> > > > /* Preempt is disabled by ftrace */
>> > > > p = get_kprobe((kprobe_opcode_t *)ip);
>> > > > if (unlikely(!p) || kprobe_disabled(p))
>> > > > return;
>> > > >
>> > > > kcb = get_kprobe_ctlblk();
>> > > > if (kprobe_running()) {
>> > > > kprobes_inc_nmissed_count(p);
>> > > > } else {
>> > > > unsigned long orig_ip = regs->ip;
>> > > > /* Kprobe handler expects regs->ip = ip + 1 as breakpoint hit */
>> > > > regs->ip = ip + sizeof(kprobe_opcode_t);
>> > > >
>> > > > /* To emulate trap based kprobes, preempt_disable here */
>> > > > preempt_disable();
>> > > > __this_cpu_write(current_kprobe, p);
>> > > > kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>> > > > if (!p->pre_handler || !p->pre_handler(p, regs)) {
>> > > > __skip_singlestep(p, regs, kcb, orig_ip);
>> > > > preempt_enable_no_resched();
>> > >
>> > > This preemption disabling and enabling looks rather strange. Looking at
>> > > git blame, it appears this was added for jprobes. Can we remove it now
>> > > that jprobes is going away?
>> >
>> > No, that is not for jprobes but for compatibility with kprobe's user
>> > handler. Since this transformation is done silently, user can not
>> > change their handler for ftrace case. So we need to keep this condition
>> > same as original kprobes.
>> >
>> > And anyway, for using smp_processor_id() for accessing per-cpu,
>> > we should disable preemption, correct?
>>
>> But as stated at the start of the function:
>>
>> /* Preempt is disabled by ftrace */
>
> Ah, yes. So this is only for the jprobes.
>
>>
>>
>> The reason I ask, is that we have for this function:
>>
>> /* To emulate trap based kprobes, preempt_disable here */
>> preempt_disable();
>> __this_cpu_write(current_kprobe, p);
>> kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>> if (!p->pre_handler || !p->pre_handler(p, regs)) {
>> __skip_singlestep(p, regs, kcb, orig_ip);
>> preempt_enable_no_resched();
>> }
>>
>> And in arch/x86/kernel/kprobes/core.c we have:
>>
>> preempt_disable();
>>
>> kcb = get_kprobe_ctlblk();
>> p = get_kprobe(addr);
>>
>> if (p) {
>> if (kprobe_running()) {
>> if (reenter_kprobe(p, regs, kcb))
>> return 1;
>> } else {
>> set_current_kprobe(p, regs, kcb);
>> kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>>
>> /*
>> * If we have no pre-handler or it returned 0, we
>> * continue with normal processing. If we have a
>> * pre-handler and it returned non-zero, it prepped
>> * for calling the break_handler below on re-entry
>> * for jprobe processing, so get out doing nothing
>> * more here.
>> */
>> if (!p->pre_handler || !p->pre_handler(p, regs))
>> setup_singlestep(p, regs, kcb, 0);
>> return 1;
>>
>>
>> Which is why I thought it was for jprobes. I'm a bit confused about
>> where preemption is enabled again.
>
> You're right. So I would like to remove it with x86 jprobe support
> code to avoid inconsistency.

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.

Thanks,
Naveen


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