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

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Date:   Tue, 7 Aug 2018 20:53:54 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and
 unify their usage

On Tue, Aug 7, 2018 at 8:44 PM, Joel Fernandes <joelaf@...gle.com> wrote:
> Hi Steve,
>
> On Tue, Aug 7, 2018 at 7:28 PM, Steven Rostedt <rostedt@...dmis.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 :-(