Re: [PATCH 4/8] sched,rcu,tracing: Mark preempt_count_{add,sub}() notrace

[Peter Zijlstra]

On Wed, Feb 12, 2020 at 10:14:15AM -0500, Steven Rostedt wrote:
> > My initial patch has __preempt_count_add/sub() in, but then I figured
> > someone would go complain the tracepoint would go missing.
> 
> Fine, but what bug are you trying to fix? I haven't seen one mentioned
> yet. Function tracing has recursion protection, and tracing
> preempt_count in nmi_enter() causes no problems. What's the problem you
> are trying to solve?

The same one as yesterday; if we hit a tracer in NMI context, when
!rcu_is_watching() and in_nmi() is still 0, we're fucked.