Re: Kernel broken on processors without performance counters

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 23, 2015 at 03:14:13PM -0400, Jason Baron wrote:
> > +static enum jump_label_type jump_label_type(struct jump_entry *entry)
> > +{
> > +	struct static_key *key = static_key_cast(iter->key);
> > +	bool true_branch = jump_label_get_branch_default(key);
> > +	bool state = static_key_enabled(key);
> > +	bool inv = static_key_inv(iter->key);
> > +
> > +	return (true_branch ^ state) ^ inv;
> 
> iiuc...this allows both the old style keys to co-exist with the
> new ones. IE state wouldn't be set for the new ones. And inv
> shouldn't be set for the old ones. Is that correct?

@state is the dynamic variable here, the other two are compile time.
@true_branch denotes the default (compile time) value, and @inv denotes
the (compile time) branch preference.

So @state will still be set for the new ones, but you're right in that
@inv will not be set for the 'legacy' stuff.

This one little line needs a big comment.