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

[Thomas Gleixner]

On Wed, 18 Oct 2017, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

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