Messages in this thread | | | Date | Wed, 18 Nov 2015 13:11:45 +0000 (UTC) | From | Mathieu Desnoyers <> | Subject | Re: trace: trace_kprobe.c always shows interrupts off |
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----- On Nov 18, 2015, at 1:51 AM, masami hiramatsu pt masami.hiramatsu.pt@hitachi.com wrote:
> Hi Mathieu, Steven, > > From: Steven Rostedt [mailto:rostedt@goodmis.org] >> >>On Tue, 17 Nov 2015 16:35:35 +0000 (UTC) >>Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote: >> >>> Hi, >>> >>> I notice that trace_kprobe.c does local_save_flags() within >>> __kprobe_trace_func(), which is called (at least on x86) with >>> interrupts always disabled. This is then used as interrupt on/off >>> state within the recorded event, which is misleading. >> >>Kinda. The kprobe itself has interrupts disabled, so it's only a white >>lie. >> >>> >>> I also don't understand why arch/x86/kernel/kprobes/ftrace.c >>> pre handler disables interrupts, considering the following >>> comment above arch/x86/kernel/kprobes/core.c: kprobe_int3_handler() >>> >>> * Interrupts are disabled on entry as trap3 is an interrupt gate and they >>> * remain disabled throughout this function. >> >>I think you answered your own question. The key piece you may be >>missing is that kprobe_ftrace_handler() is not called from a trap, but >>from a function traced callback, which does not disable interrupts. > > right, since the user can not forcibly change any kprobes to > jump, those handlers should be ran under the same environment. > >>> A struct pt_regs is received by this function, but I don't see >>> any way to get the state of irq enable/disable from struct pt_regs >>> across architectures. >>> >>> Any thoughts on how to fix this ? >> >>Create a cross arch: flags = regs_irq_save(regs) function. >> > > Good idea! Anyway, we can start on x86, in other arch we can > prepare a dummy function to return current interrupt state(as > we are doing now).
Derived from Steven's idea, I added a lttng_regs_irqs_disabled(struct pt_regs *regs) to lttng-modules. The generic version returns "-1", which means that the architecture don't support it yet. It returns 1 or 0 when implemented.
I prefer this approach compared to fall-back to irqs_disabled(), because users then know that they can trust the information when it is available.
Returning an unknown state with a regs_irq_save(regs) did not seem obvious, because there is no guarantee that a value like "-1" does not mean something specific on some architectures. One possibility would be to make it return success/failure, and populate the flags in an output parameter, but that seems rather cumbersome.
When regs_irq_save() becomes widely available across architectures in newer kernels, I'll use it of course.
Thanks,
Mathieu
> > Thank you, > > ---- > Masami Hiramatsu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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