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Date: Sat, 05 May 2018 13:16:04 +0530 From: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> To: Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, linux-trace-users@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, Namhyung Kim <namhyung@...nel.org>, Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>, shuah@...nel.org, Tom Zanussi <tom.zanussi@...ux.intel.com> Subject: Re: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features Masami Hiramatsu wrote: > On Fri, 4 May 2018 12:06:42 -0400 > Steven Rostedt <rostedt@...dmis.org> wrote: > >> On Sat, 5 May 2018 00:48:28 +0900 >> Masami Hiramatsu <mhiramat@...nel.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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