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

[Paul E. McKenney]

On Wed, Aug 08, 2018 at 08:46:29AM -0400, Steven Rostedt wrote:
> On Tue, 7 Aug 2018 20:53:54 -0700
> Joel Fernandes <joelaf@xxxxxxxxxx> 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?

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.

							Thanx, Paul