Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

[Andy Lutomirski]

> On Mar 1, 2020, at 11:30 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> 
> ïOn 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@xxxxxxxxxx> wrote:
>>> 
>>> On Sun, Mar 01, 2020 at 07:12:25PM +0100, Thomas Gleixner wrote:
>>>> Andy Lutomirski <luto@xxxxxxxxxx> writes:
>>>>> On Sun, Mar 1, 2020 at 7:21 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>>>>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>>>>>>> On Mar 1, 2020, at 2:16 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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 donât suppose we could make notrace imply nokprobe.  Then all kprobeable functions would also have entry/exit tracepoints, right?