linux-kernel - Re: Kernel broken on processors without performance counters

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Date:	Tue, 21 Jul 2015 17:49:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Jason Baron <jasonbaron0@...il.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Vince Weaver <vince@...ter.net>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel broken on processors without performance counters

On Tue, Jul 21, 2015 at 05:43:27PM +0200, Thomas Gleixner wrote:
> On Tue, 21 Jul 2015, Peter Zijlstra wrote:
> 
> > > -#endif	/* _LINUX_JUMP_LABEL_H */
> > > +static inline void static_key_enable(struct static_key *key)
> > > +{
> > > +	int count = static_key_count(key);
> > > +
> > > +	WARN_ON_ONCE(count < 0 || count > 1);
> > > +
> > > +	if (!count)
> > > +		static_key_slow_inc(key);
> > > +}
> > > +
> > > +static inline void static_key_disable(struct static_key *key)
> > > +{
> > > +	int count = static_key_count(key);
> > > +
> > > +	WARN_ON_ONCE(count < 0 || count > 1);
> > > +
> > > +	if (count)
> > > +		static_key_slow_dec(key);
> > > +}
> 
> The functions above are not part of the interface which should be used
> in code, right?

They are in fact. They make it easier to deal with the refcount thing
when all you want is boolean states.

> > > +/* -------------------------------------------------------------------------- */
> > > +
> > > +/*
> > > + * likely -- default enabled, puts the branch body in-line
> > > + */
> > > +
> > > +struct static_likely_key {
> > > +	struct static_key key;
> > > +};
> > > +
> > > +#define STATIC_LIKELY_KEY_INIT	(struct static_likely_key){ .key = STATIC_KEY_INIT_TRUE, }
> > > +
> > > +static inline bool static_likely_branch(struct static_likely_key *key)
> > > +{
> > > +	return static_key_true(&key->key);
> > > +}
> > > +
> > > +/*
> > > + * unlikely -- default disabled, puts the branch body out-of-line
> > > + */
> > > +
> > > +struct static_unlikely_key {
> > > +	struct static_key key;
> > > +};
> > > +
> > > +#define STATIC_UNLIKELY_KEY_INIT (struct static_unlikely_key){ .key = STATIC_KEY_INIT_FALSE, }
> > > +
> > > +static inline bool static_unlikely_branch(struct static_unlikely_key *key)
> > > +{
> > > +	return static_key_false(&key->key);
> > > +}
> > > +
> > > +/*
> > > + * Advanced usage; refcount, branch is enabled when: count != 0
> > > + */
> > > +
> > > +#define static_branch_inc(_k)	static_key_slow_inc(&(_k)->key)
> > > +#define static_branch_dec(_k)	static_key_slow_dec(&(_k)->key)
> 
> Inlines please
> 
> > > +/*
> > > + * Normal usage; boolean enable/disable.
> > > + */
> > > +
> > > +#define static_branch_enable(_k)	static_key_enable(&(_k)->key)
> > > +#define static_branch_disable(_k)	static_key_disable(&(_k)->key)
> 
> Ditto
> 
> All in all much more understandable than the existing horror.

They cannot in fact be inlines because we play type tricks. Note how _k
can be either struct static_likely_key or struct static_unlikely_key.


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