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

[Steven Rostedt]

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.

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

-- Steve