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

[Paul E. McKenney]

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@xxxxxxxxxxxxxxxxxx> 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.

> 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