Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage

[Joel Fernandes <joelaf@google.com>]

On Wed, Aug 8, 2018 at 6:00 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Tue, Aug 07, 2018 at 08:53:54PM -0700, Joel Fernandes wrote:
>> On Tue, Aug 7, 2018 at 8:44 PM, Joel Fernandes <joelaf@google.com> wrote:
>> > Hi Steve,
>> >
>> > On Tue, Aug 7, 2018 at 7:28 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
>> [...]
>> >>> @@ -171,8 +174,7 @@ extern void syscall_unregfunc(void);
>> >>>                       } while ((++it_func_ptr)->func);                \
>> >>>               }                                                       \
>> >>>                                                                       \
>> >>> -             if (rcuidle)                                            \
>> >>> -                     srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
>> >>> +             srcu_read_unlock_notrace(ss, idx);                      \
>> >>
>> >> Hmm, why do we have the two different srcu handles?
>> >
>> > Because if the memory operations happening on the normal SRCU handle
>> > (during srcu_read_lock) is interrupted by NMI, then the other handle
>> > (devoted to NMI) could be used instead and not bother the interrupted
>> > handle. Does that makes sense?
>> >
>> > When I talked to Paul few months ago about SRCU from NMI context, he
>> > mentioned the per-cpu memory operations during srcu_read_lock can be
>> > NMI interrupted, that's why we added that warning.
>>
>> So I looked more closely, __srcu_read_lock on 2 different handles may
>> still be doing a this_cpu_inc on the same location..
>> (sp->sda->srcu_lock_count). :-(
>>
>> Paul any ideas on how to solve this?
>
> You lost me on this one.  When you said "2 different handles", I assumed
> that you meant two different values of "sp", which would have two
> different addresses for &sp->sda->srcu_lock_count.  What am I missing?

Thanks a lot for the reply.
I thought "sda" is the same for different srcu_struct(s). May be it
was too late for me in the night, that's why I thought so? Which makes
no sense now that I think of it.

In that case based on what you're saying, the patch I sent to using
different srcu_struct for NMI is still good I guess...

>> It does start to seem like a show stopper :-(
>
> I suppose that an srcu_read_lock_nmi() and srcu_read_unlock_nmi() could
> be added, which would do atomic ops on sp->sda->srcu_lock_count.  Not sure
> whether this would be fast enough to be useful, but easy to provide:
>
> int __srcu_read_lock_nmi(struct srcu_struct *sp)  /* UNTESTED. */
> {
>         int idx;
>
>         idx = READ_ONCE(sp->srcu_idx) & 0x1;
>         atomic_inc(&sp->sda->srcu_lock_count[idx]);
>         smp_mb__after_atomic(); /* B */  /* Avoid leaking critical section. */
>         return idx;
> }
>
> void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx)
> {
>         smp_mb__before_atomic(); /* C */  /* Avoid leaking critical section. */
>         atomic_inc(&sp->sda->srcu_unlock_count[idx]);
> }
>
> With appropriate adjustments to also allow Tiny RCU to also work.
>
> Note that you have to use _nmi() everywhere, not just in NMI handlers.
> In fact, the NMI handlers are the one place you -don't- need to use
> _nmi(), strangely enough.
>
> Might be worth a try -- smp_mb__{before,after}_atomic() is a no-op on
> some architectures, for example.

Continuing Steve's question on regular interrupts, do we need to use
this atomic_inc API for regular interrupts as well?

Thanks!