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Date:   Wed, 8 Aug 2018 09:07:24 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     Joel Fernandes <joelaf@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        LKML <linux-kernel@vger.kernel.org>,
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        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>,
        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 Wed, 8 Aug 2018 06:03:02 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

>  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?  
> 
> On architectures without increment-memory instructions, if you take an NMI
> between the load from sp->sda->srcu_lock_count and the later store, you
> lose a count.  Note that both __srcu_read_lock() and __srcu_read_unlock()
> do increments of different locations, so you cannot rely on the usual
> "NMI fixes up before exit" semantics you get when incrementing and
> decrementing the same location.

And how is this handled in the interrupt case? Interrupts are not
disabled here.

I would also argue that architectures without increment-memory
instructions shouldn't have NMIs ;-)

-- Steve