Re: [PATCH 1/2] lockdep: Introduce CROSSRELEASE_STACK_TRACE and make it not unwind as default

[Byungchul Park <byungchul.park@lge.com>]

On Wed, Oct 18, 2017 at 03:36:05PM +0200, Thomas Gleixner wrote:
> On Wed, 18 Oct 2017, Ingo Molnar wrote:
> > * Thomas Gleixner <tglx@linutronix.de> wrote:
> > 
> > > On Wed, 18 Oct 2017, Byungchul Park wrote:
> > > >  #ifdef CONFIG_LOCKDEP_CROSSRELEASE
> > > > +#ifdef CONFIG_CROSSRELEASE_STACK_TRACE
> > > >  #define MAX_XHLOCK_TRACE_ENTRIES 5
> > > > +#else
> > > > +#define MAX_XHLOCK_TRACE_ENTRIES 1
> > > > +#endif
> > > >  
> > > >  /*
> > > >   * This is for keeping locks waiting for commit so that true dependencies
> > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > > index e36e652..5c2ddf2 100644
> > > > --- a/kernel/locking/lockdep.c
> > > > +++ b/kernel/locking/lockdep.c
> > > > @@ -4863,8 +4863,13 @@ static void add_xhlock(struct held_lock *hlock)
> > > >  	xhlock->trace.nr_entries = 0;
> > > >  	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
> > > >  	xhlock->trace.entries = xhlock->trace_entries;
> > > > +#ifdef CONFIG_CROSSRELEASE_STACK_TRACE
> > > >  	xhlock->trace.skip = 3;
> > > >  	save_stack_trace(&xhlock->trace);
> > > > +#else
> > > > +	xhlock->trace.nr_entries = 1;
> > > > +	xhlock->trace.entries[0] = hlock->acquire_ip;
> > > > +#endif
> > > 
> > > Hmm. Would it be possible to have this switchable at boot time via a
> > > command line parameter? So in case of a splat with no stack trace, one
> > > could just reboot and set something like 'lockdep_fullstack' on the kernel
> > > command line to get the full data without having to recompile the kernel.
> > 
> > Yeah, and I'd suggest keeping the Kconfig option to default-enable that boot 
> > option as well - i.e. let's have both.
> 
> That makes sense. Like we have with debug objects:
> DEBUG_OBJECTS_ENABLE_DEFAULT.

Thank you very much for the suggestion. I will work for it.

> Which reminds me that I wanted to convert them to static_key so they are
> zero overhead when disabled. Sigh, why are todo lists growth only?
> 
> Thanks,
> 
> 	tglx