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

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Message-ID: <20180808104910.2ced5e51@gandalf.local.home>
Date:   Wed, 8 Aug 2018 10:49:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        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>,
        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 Wed, 8 Aug 2018 07:33:10 -0700
"Paul E. McKenney" <paulmck@...ux.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@...ux.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?

-- Steve

> 
> > I would also argue that architectures without increment-memory
> > instructions shouldn't have NMIs ;-)  
> 
> I would also argue a lot of things, but objective reality does not take my
> opinions into account all that often.  Which might be a good thing.  ;-)
> 
> 							Thanx, Paul