Re: [PATCH 13/14] lockdep: Prepare for NMI IRQ state tracking

[Peter Zijlstra]

On Fri, May 29, 2020 at 06:14:01PM -0400, Steven Rostedt wrote:
> On Fri, 29 May 2020 23:27:41 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > There is no reason not to always, accurately, track IRQ state.
> > 
> > This change also makes IRQ state tracking ignore lockdep_off().
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> >  kernel/locking/lockdep.c |   33 ++++++++++++++++++++++++++++++---
> >  1 file changed, 30 insertions(+), 3 deletions(-)
> > 
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -3646,7 +3646,13 @@ static void __trace_hardirqs_on_caller(v
> >   */
> >  void lockdep_hardirqs_on_prepare(unsigned long ip)
> >  {
> > -	if (unlikely(!debug_locks || current->lockdep_recursion))
> 
> Why remove the check for debug_locks? Isn't that there to disable
> everything at once to prevent more warnings to be printed?

Yeah, maybe. I was thinking we could keep IRQ state running. But you're
right, if we mess up the IRQ state itself this might generate a wee
mess.

> Also, isn't there other ways that we could have recursion besides NMIs?
> Say we do a printk inside here, or call something that may also enable
> interrupts? I thought the recursion check was also to prevent lockdep
> infrastructure calling something that lockdep monitors being a problem?
> 
> Or am I missing something?

> > +	/*
> > +	 * NMIs do not (and cannot) track lock dependencies, nothing to do.
> > +	 */
> > +	if (in_nmi())
> > +		return;
> > +
> > +	if (DEBUG_LOCKS_WARN_ON(current->lockdep_recursion & LOCKDEP_RECURSION_MASK))
> >  		return;

^^ there's your regular recursion check.