Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature

[Peter Zijlstra]

On Tue, Jan 17, 2017 at 11:05:42AM +0900, Byungchul Park wrote:
> On Mon, Jan 16, 2017 at 04:10:01PM +0100, Peter Zijlstra wrote:
> > On Fri, Dec 09, 2016 at 02:12:03PM +0900, Byungchul Park wrote:

> > > @@ -155,6 +164,9 @@ struct lockdep_map {
> > >  	int				cpu;
> > >  	unsigned long			ip;
> > >  #endif
> > > +#ifdef CONFIG_LOCKDEP_CROSSRELEASE
> > > +	struct cross_lock		*xlock;
> > > +#endif
> > 
> > The use of this escapes me; why does the lockdep_map need a pointer to
> > this?
> 
> Lockdep interfaces e.g. lock_acquire(), lock_release() and lock_commit()
> use lockdep_map as an arg, but crossrelease need to extract cross_lock
> instances from that.

> > Why not do something like:
> > 
> > struct lockdep_map_cross {
> > 	struct lockdep_map	map;
> > 	struct held_lock	hlock;
> > }

Using a structure like that, you can pass lockdep_map_cross around just
fine, since the lockdep_map is the first member, so the pointers are
interchangeable. At worst we might need to munge a few typecasts.

But then the cross release code can simply cast to the bigger type and
have access to the extra data it knows to be there.