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SubjectRe: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code
On Sun, 1 Mar 2020 11:39:42 -0800
Andy Lutomirski <luto@amacapital.net> wrote:

> > On Mar 1, 2020, at 11:30 AM, Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Sun, Mar 01, 2020 at 10:54:23AM -0800, Andy Lutomirski wrote:
> >>> On Sun, Mar 1, 2020 at 10:26 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >>>
> >>> On Sun, Mar 01, 2020 at 07:12:25PM +0100, Thomas Gleixner wrote:
> >>>> Andy Lutomirski <luto@kernel.org> writes:
> >>>>> On Sun, Mar 1, 2020 at 7:21 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >>>>>> Andy Lutomirski <luto@amacapital.net> writes:
> >>>>>>>> On Mar 1, 2020, at 2:16 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >>>>>>>> Ok, but for the time being anything before/after CONTEXT_KERNEL is unsafe
> >>>>>>>> except trace_hardirq_off/on() as those trace functions do not allow to
> >>>>>>>> attach anything AFAICT.
> >>>>>>>
> >>>>>>> Can you point to whatever makes those particular functions special? I
> >>>>>>> failed to follow the macro maze.
> >>>>>>
> >>>>>> Those are not tracepoints and not going through the macro maze. See
> >>>>>> kernel/trace/trace_preemptirq.c
> >>>>>
> >>>>> That has:
> >>>>>
> >>>>> void trace_hardirqs_on(void)
> >>>>> {
> >>>>> if (this_cpu_read(tracing_irq_cpu)) {
> >>>>> if (!in_nmi())
> >>>>> trace_irq_enable_rcuidle(CALLER_ADDR0, CALLER_ADDR1);
> >>>>> tracer_hardirqs_on(CALLER_ADDR0, CALLER_ADDR1);
> >>>>> this_cpu_write(tracing_irq_cpu, 0);
> >>>>> }
> >>>>>
> >>>>> lockdep_hardirqs_on(CALLER_ADDR0);
> >>>>> }
> >>>>> EXPORT_SYMBOL(trace_hardirqs_on);
> >>>>> NOKPROBE_SYMBOL(trace_hardirqs_on);
> >>>>>
> >>>>> But this calls trace_irq_enable_rcuidle(), and that's the part of the
> >>>>> macro maze I got lost in. I found:
> >>>>>
> >>>>> #ifdef CONFIG_TRACE_IRQFLAGS
> >>>>> DEFINE_EVENT(preemptirq_template, irq_disable,
> >>>>> TP_PROTO(unsigned long ip, unsigned long parent_ip),
> >>>>> TP_ARGS(ip, parent_ip));
> >>>>>
> >>>>> DEFINE_EVENT(preemptirq_template, irq_enable,
> >>>>> TP_PROTO(unsigned long ip, unsigned long parent_ip),
> >>>>> TP_ARGS(ip, parent_ip));
> >>>>> #else
> >>>>> #define trace_irq_enable(...)
> >>>>> #define trace_irq_disable(...)
> >>>>> #define trace_irq_enable_rcuidle(...)
> >>>>> #define trace_irq_disable_rcuidle(...)
> >>>>> #endif
> >>>>>
> >>>>> But the DEFINE_EVENT doesn't have the "_rcuidle" part. And that's
> >>>>> where I got lost in the macro maze. I looked at the gcc asm output,
> >>>>> and there is, indeed:
> >>>>
> >>>> DEFINE_EVENT
> >>>> DECLARE_TRACE
> >>>> __DECLARE_TRACE
> >>>> __DECLARE_TRACE_RCU
> >>>> static inline void trace_##name##_rcuidle(proto)
> >>>> __DO_TRACE
> >>>> if (rcuidle)
> >>>> ....
> >>>>
> >>>>> But I also don't see why this is any different from any other tracepoint.
> >>>>
> >>>> Indeed. I took a wrong turn at some point in the macro jungle :)
> >>>>
> >>>> So tracing itself is fine, but then if you have probes or bpf programs
> >>>> attached to a tracepoint these use rcu_read_lock()/unlock() which is
> >>>> obviosly wrong in rcuidle context.
> >>>
> >>> Definitely, any such code needs to use tricks similar to that of the
> >>> tracing code. Or instead use something like SRCU, which is OK with
> >>> readers from idle. Or use something like Steve Rostedt's workqueue-based
> >>> approach, though please be very careful with this latter, lest the
> >>> battery-powered embedded guys come after you for waking up idle CPUs
> >>> too often. ;-)
> >>
> >> Are we okay if we somehow ensure that all the entry code before
> >> enter_from_user_mode() only does rcuidle tracing variants and has
> >> kprobes off? Including for BPF use cases?
> >
> > That would work, though if BPF used SRCU instead of RCU, this would
> > be unnecessary. Sadly, SRCU has full memory barriers in each of
> > srcu_read_lock() and srcu_read_unlock(), but we are working on it.
> > (As always, no promises!)
> >
> >> It would be *really* nice if we could statically verify this, as has
> >> been mentioned elsewhere in the thread. It would also probably be
> >> good enough if we could do it at runtime. Maybe with lockdep on, we
> >> verify rcu state in tracepoints even if the tracepoint isn't active?
> >> And we could plausibly have some widget that could inject something
> >> into *every* kprobeable function to check rcu state.
> >
> > Or just have at least one testing step that activates all tracepoints,
> > but with lockdep enabled?
>
> Also kprobe.
>
> I don’t suppose we could make notrace imply nokprobe. Then all
> kprobeable functions would also have entry/exit tracepoints, right?

There was some code before that prevented a kprobe from being allowed
in something that was not in the ftrace mcount table (which would make
this happen). But I think that was changed because it was too
restrictive.

Masami?

-- Steve

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