Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change

[Peter Zijlstra]

On Wed, May 29, 2019 at 09:42:13AM -0400, Steven Rostedt wrote:
> > And the preempt_irqoff tracer had better also consume the IRQ events,
> > and if it does that it can DTRT without extra bits on, even with that
> > race.
> > 
> > Consider:
> > 
> > 	preempt_disable()
> > 	  preempt_count += 1;
> > 	  <IRQ>
> > 	    trace_irq_enter();
> > 
> > 	    trace_irq_exit();
> > 	  </IRQ>
> > 	  trace_preempt_disable();
> > 
> > 	/* does stuff */
> > 
> > 	preempt_enable()
> > 	  preempt_count -= 1;
> > 	  trace_preempt_enable();
> > 
> > You're saying preempt_irqoff() fails to connect the two because of the
> > hole between trace_irq_exit() and trace_preempt_disable() ?
> > 
> > But trace_irq_exit() can see the raised preempt_count and set state
> > for trace_preempt_disable() to connect.
> 
> That's basically what I was suggesting as the solution to this ;-)

You were wanting changes to preempt_disable() and task_struct, neither
of which is required. The above only needs some per-cpu storage in the
tracer implementation.