Date:   Sun, 1 Mar 2020 12:18:07 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to
 C-code

On Sun, Mar 01, 2020 at 11:39:42AM -0800, Andy Lutomirski wrote:
> 
> 
> > On Mar 1, 2020, at 11:30 AM, Paul E. McKenney <paulmck@...nel.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@...nel.org> wrote:
> >>> 
> >>> On Sun, Mar 01, 2020 at 07:12:25PM +0100, Thomas Gleixner wrote:
> >>>> Andy Lutomirski <luto@...nel.org> writes:
> >>>>> On Sun, Mar 1, 2020 at 7:21 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >>>>>> Andy Lutomirski <luto@...capital.net> writes:
> >>>>>>>> On Mar 1, 2020, at 2:16 AM, Thomas Gleixner <tglx@...utronix.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 didn't realize we could test all kprobe points easily, but yes,
that would also be good.

> I don’t suppose we could make notrace imply nokprobe.  Then all kprobeable functions would also have entry/exit tracepoints, right?

On this, I must defer to the tracing and kprobes people.

							Thanx, Paul

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