%% use our own settings
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%deffont "title"	xfont "P22 Typewriter"
#%deffont "thick"	xfont "bitstream vera sans:bold"
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%% Default settings per each line numbers.
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%default 2 size 10, vgap 10, prefix " ", fore "black", font "title"
#%default 3 size 2, bar "gray70", vgap 10, fore "black"
%default 3 size 2, bar "steel blue", vgap 10, fore "black"
%default 4 size 8, fore "white", vgap 10, prefix " ", font "standard", fore "black"
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
%nodefault
%fore "black", back "white", center

%size 30, font "title"
#%font "typewriter"
struct kref
%size 1


%size 10
Reference counting made easy

%font "standard"
%size 8

Greg Kroah-Hartman
%size 7
%font "email"
greg@kroah.com
gregkh@us.ibm.com
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
%nodefault
%size 8, font "title", fore "black", back "white", leftfill



%font "standard", size 8
        "Remember: if another thread can find your data 
         structure, and you don't have a reference count on 
         it, you almost certainly have a bug."
%font "typewriter", size 6
                             -Documentation/CodingStyle
 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

Reference counting is essential


	keeps memory leaks from happening

	protects from accessing freed memory
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

What is a reference count


	an atomic counter

	not a lock
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

How to use it


	embed a 
%cont, font "typewriter"
struct kref
%cont, font "standard"
 in your structure:

%font "typewriter"
      struct foo {
              int baz;
              struct kref kref;
              ...
      };
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

How to use it (cont.)


	allocate an instance:
%font "typewriter"
     foo = kmalloc(sizeof(*foo), GFP_KERNEL);
%font "standard"

	initialize the kref:
%font "typewriter"
     kref_init(&foo->kref);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

How to use it (cont.)


	increment the count before giving it to anyone else:
%font "typewriter"
     kref_get(&foo->kref);
%font "standard"

	decrement the count when you are finished with it:
%font "typewriter"
     kref_put(&foo->kref, foo_release);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

release


	called when last reference is freed.

	must look like:
%font "typewriter"
     void foo_release(struct kref *kref);
%font "standard"
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

example release


%font "typewriter"
void foo_release(struct kref *kref)
{
        struct foo *foo;

        foo = container_of(foo, struct foo,
                           kref);
        kfree(foo);
}
%font "standard"

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

3 rules of use


	
%cont, font "typewriter"
kref_get()
%cont, font "standard"
 before giving to others

	after 
%cont, font "typewriter"
kref_put()
%cont, font "standard"
 do not touch

	serialize last expected 
%cont, font "typewriter"
kref_put()
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page

Future stuff


	rcu version of 
%cont, font "typewriter"
kref_get()
%cont, font "standard"
 and 
%cont, font "typewriter"
kref_put()
%font "standard"
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
%nodefault
%fore "black", back "white", center

%size 30, font "title"
#%font "typewriter"
struct kref
%size 1


%size 10
Reference counting made easy

%font "standard"
%size 8

Greg Kroah-Hartman
%size 7
%font "email"
greg@kroah.com
gregkh@us.ibm.com
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%