linux-kernel - Re: [PATCH 2/2] jump label: introduce static

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Date:	Wed, 5 Jan 2011 16:14:44 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	peterz@...radead.org, mathieu.desnoyers@...ymtl.ca, hpa@...or.com,
	rostedt@...dmis.org, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, avi@...hat.com,
	davem@...emloft.net, sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org
Subject: Re: [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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