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

[Joel Fernandes <joelaf@google.com>]

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?

It does start to seem like a show stopper :-(