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SubjectRe: [PATCH 2/2] jump label: introduce static_branch()
On Wed, Jan 05, 2011 at 06:15:18PM +0100, Frederic Weisbecker wrote:
> On Wed, Jan 05, 2011 at 10:43:12AM -0500, Jason Baron wrote:
> > diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> > index 152f7de..0ad9c2e 100644
> > --- a/include/linux/jump_label.h
> > +++ b/include/linux/jump_label.h
> > @@ -22,6 +22,11 @@ struct module;
> >
> > #ifdef HAVE_JUMP_LABEL
> >
> > +static __always_inline bool static_branch(struct jump_label_key *key)
> > +{
> > + return __static_branch(key);
>
> Not very important, but __static_branch() would be more self-explained
> if it was called arch_static_branch().
>
> > +}
> > +
> > extern struct jump_entry __start___jump_table[];
> > extern struct jump_entry __stop___jump_table[];
> >
> > @@ -42,11 +47,12 @@ struct jump_label_key {
> > int state;
> > };
> >
> > -#define JUMP_LABEL(key, label) \
> > -do { \
> > - if (unlikely(((struct jump_label_key *)key)->state)) \
> > - goto label; \
> > -} while (0)
> > +static __always_inline bool static_branch(struct jump_label_key *key)
> > +{
> > + if (unlikely(key->state))
> > + return true;
> > + return false;
> > +}
> >
> > static inline int jump_label_enabled(struct jump_label_key *key)
> > {
> > @@ -78,14 +84,4 @@ static inline void jump_label_unlock(void) {}
> >
> > #endif
> >
> > -#define COND_STMT(key, stmt) \
> > -do { \
> > - __label__ jl_enabled; \
> > - JUMP_LABEL_ELSE_ATOMIC_READ(key, jl_enabled); \
> > - if (0) { \
> > -jl_enabled: \
> > - stmt; \
> > - } \
> > -} while (0)
> > -
> > #endif
> > diff --git a/include/linux/jump_label_ref.h b/include/linux/jump_label_ref.h
> > index 8a76e89..5178696 100644
> > --- a/include/linux/jump_label_ref.h
> > +++ b/include/linux/jump_label_ref.h
> > @@ -7,19 +7,23 @@
> > struct jump_label_key_counter {
> > atomic_t ref;
> > struct jump_label_key key;
> > -}
> > +};
> >
> > #ifdef HAVE_JUMP_LABEL
> >
> > -#define JUMP_LABEL_ELSE_ATOMIC_READ(key, label, counter) JUMP_LABEL(key, label)
> > +static __always_inline bool static_branch_else_atomic_read(struct jump_label_key *key, atomic_t *count)
> > +{
> > + return __static_branch(key);
> > +}
>
> How about having only static_branch() but the key would be handled only
> by ways of get()/put().
>
> Simple boolean key enablement would work in this scheme as well as branches
> based on refcount. So that the users could avoid maintaining both key and count,
> this would be transparently handled by the jump label API.
>
> Or am I missing something?
>

right. this is a good point. I had a 'jump_label_inc()',
'jump_label_dec()' essentially providing this. However, when jump labels
are disabled we didn't want to incur an atomic_read() everywhere.
Furthermore, the use of the atomic_t type within jump_label.h, causes
#include dependencies problems, since atomic.h ends up including
jump_label.h...

Thus, what I've proposed here, is to the have the very simple
jump_label_enable()/disable(), and leave reference counting to the
caller.

thanks,

-Jason


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