From mboxrd@z Thu Jan 1 00:00:00 1970 From: mhiramat@kernel.org (Masami Hiramatsu) Date: Sat, 5 May 2018 23:32:11 +0900 Subject: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features In-Reply-To: <1525506231.y43xvmd3yw.naveen@linux.ibm.com> References: <152465856498.26224.16969986455942749517.stgit@devbox> <20180503181137.6d82d897@gandalf.local.home> <20180505004828.9b75b6802472f09b0d2de5b8@kernel.org> <20180504120642.354cdd1f@gandalf.local.home> <20180505113803.45700179a93091b114ec6984@kernel.org> <1525506231.y43xvmd3yw.naveen@linux.ibm.com> Message-ID: <20180505233211.ecad929b13461f1e5f6427d2@kernel.org> Content-Type: text/plain; charset="UTF-8" Message-ID: <20180505143211.cPeHBgtHkzS0lmiy1mhSP35ppjgCIVP2m5iVFRM0GZg@z> On Sat, 05 May 2018 13:16:04 +0530 "Naveen N. Rao" wrote: > Masami Hiramatsu wrote: > > On Fri, 4 May 2018 12:06:42 -0400 > > Steven Rostedt wrote: > > > >> On Sat, 5 May 2018 00:48:28 +0900 > >> Masami Hiramatsu 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. 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. Thank you, -- Masami Hiramatsu -- To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in the body of a message to majordomo at vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html