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

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Date:	Tue, 21 Jul 2015 08:51:51 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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 8:49 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.
>
>

To clarify my (mis-)understanding:

There are two degrees of freedom in a static_key.  They can start out
true or false, and they can be unlikely or likely.  Are those two
degrees of freedom in fact tied together?

--Andy
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