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Date:   Wed, 8 Aug 2018 08:46:29 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Joel Fernandes <joelaf@google.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Cc: Android Kernel" <kernel-team@android.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Byungchul Park <byungchul.park@lge.com>,
        Ingo Molnar <mingo@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Glexiner <tglx@linutronix.de>,
        Tom Zanussi <tom.zanussi@linux.intel.com>
Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and
 unify their usage
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On Tue, 7 Aug 2018 20:53:54 -0700
Joel Fernandes <joelaf@google.com> wrote:


> > 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 :-(

What's wrong with a this_cpu_inc()? It's atomic for the CPU. Although
it wont be atomic for the capture of the idx. But I also don't see
interrupts being disabled, thus an NMI is no different than any
interrupt doing the same thing, right?

-- Steve