Re: [PATCH 22/27] locking/lockdep: Reuse list entries that are no longer in use

[Peter Zijlstra]

On Thu, Nov 29, 2018 at 11:49:02AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 28, 2018 at 03:43:20PM -0800, Bart Van Assche wrote:
> > Instead of abandoning elements of list_entries[] that are no longer in
> > use, make alloc_list_entry() reuse array elements that have been freed.
> 
> > diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> > index 43327a1dd488..01e55fca7c2c 100644
> > --- a/include/linux/lockdep.h
> > +++ b/include/linux/lockdep.h
> > @@ -183,6 +183,11 @@ static inline void lockdep_copy_map(struct lockdep_map *to,
> >  struct lock_list {
> >  	/* Entry in locks_after or locks_before. */
> >  	struct list_head		lock_order_entry;
> > +	/*
> > +	 * Entry in all_list_entries when in use and entry in
> > +	 * free_list_entries when not in use.
> > +	 */
> > +	struct list_head		alloc_entry;
> >  	struct lock_class		*class;
> >  	struct lock_class		*links_to;
> >  	struct stack_trace		trace;
> 
> > +static LIST_HEAD(all_list_entries);
> > +static LIST_HEAD(free_list_entries);
> >  
> 
> > @@ -862,7 +867,10 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
> >   */
> >  static struct lock_list *alloc_list_entry(void)
> >  {
> > -	if (nr_list_entries >= MAX_LOCKDEP_ENTRIES) {
> > +	struct lock_list *e = list_first_entry_or_null(&free_list_entries,
> > +						       typeof(*e), alloc_entry);
> > +
> > +	if (!e) {
> >  		if (!debug_locks_off_graph_unlock())
> >  			return NULL;
> >  
> > @@ -870,7 +878,8 @@ static struct lock_list *alloc_list_entry(void)
> >  		dump_stack();
> >  		return NULL;
> >  	}
> > -	return list_entries + nr_list_entries++;
> > +	list_move_tail(&e->alloc_entry, &all_list_entries);
> > +	return e;
> >  }
> 
> > @@ -4235,19 +4244,19 @@ static void zap_class(struct list_head *zapped_classes,
> >  		      struct lock_class *class)
> >  {
> >  	struct lock_class *links_to;
> > +	struct lock_list *entry, *tmp;
> >  
> >  	/*
> >  	 * Remove all dependencies this lock is
> >  	 * involved in:
> >  	 */
> > +	list_for_each_entry_safe(entry, tmp, &all_list_entries, alloc_entry) {
> >  		if (entry->class != class && entry->links_to != class)
> >  			continue;
> >  		links_to = entry->links_to;
> >  		WARN_ON_ONCE(entry->class == links_to);
> >  		list_del_rcu(&entry->lock_order_entry);
> > +		list_move(&entry->alloc_entry, &free_list_entries);
> >  		entry->class = NULL;
> >  		entry->links_to = NULL;
> >  		check_free_class(zapped_classes, class);
> 
> Hurm.. I'm confused here.
> 
> The reason you cannot re-use lock_order_entry for the free list is
> because list_del_rcu(), right? But if so, then what ensures the
> list_entry is not re-used before it's grace-period?

Also; if you have to grow lock_list by 16 bytes just to be able to free
it, a bitmap allocator is much cheaper, space wise.

Some people seem to really care about the static image size, and
lockdep's .data section does matter to them.