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Date:   Wed, 8 Aug 2018 08:05:58 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Joel Fernandes <joelaf@google.com>,
        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>,
        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
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Aug 08, 2018 at 10:49:10AM -0400, Steven Rostedt wrote:
> On Wed, 8 Aug 2018 07:33:10 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > On Wed, Aug 08, 2018 at 09:07:24AM -0400, Steven Rostedt wrote:
> > > 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.  
> > 
> > Actually, on most architectures interrupts are in fact disabled:
> > 
> > #define this_cpu_generic_to_op(pcp, val, op)				\
> > do {									\
> > 	unsigned long __flags;						\
> > 	raw_local_irq_save(__flags);					\
> > 	raw_cpu_generic_to_op(pcp, val, op);				\
> > 	raw_local_irq_restore(__flags);					\
> > } while (0)
> > 
> > NMIs, not so much.
> 
> And do these archs have NMIs?

It would appear so:

$ find . -name 'Kconfig*' -exec grep -l 'select HAVE_NMI\>' {} \;
./arch/sparc/Kconfig
./arch/s390/Kconfig
./arch/arm/Kconfig
./arch/arm64/Kconfig
./arch/mips/Kconfig
./arch/sh/Kconfig
./arch/powerpc/Kconfig
./arch/x86/Kconfig

							Thanx, Paul