Re: [PATCH 1/5] pid: Implement task references.

[Eric Dumazet]

Greg KH a écrit :
> On Sun, Jan 29, 2006 at 02:58:51PM -0700, Eric W. Biederman wrote:
> > Greg KH <greg@xxxxxxxxx> writes:
> >
> > > On Sun, Jan 29, 2006 at 12:22:34AM -0700, Eric W. Biederman wrote:
> > > > +struct task_ref
> > > > +{
> > > > +	atomic_t count;
> > > Please use a struct kref here, instead of your own atomic_t, as that's
> > > why it is in the kernel :)
> > >
> > > > +	enum pid_type type;
> > > > +	struct task_struct *task;
> > > > +};
> > > thanks,
> > I would rather not. Whenever I look at struct kref it seems to be an over
> > abstraction, and as such I find it confusing to work with.  I know
> > whenever I look at the sysfs code I have to actively remind myself
> > that the kref in the structure is not a pointer to a kref.
> >
> > What does the kref abstraction buy?  How does it simplify things?
> > We already have equivalent functions in atomic_t on which it is built.
>
> It ensures that you get the logic of the reference counting correctly.
> It forces you to do the logic of the get and put and release properly.
>
> To roughly quote Andrew Morton, "When I see a kref, I know it is used
> properly, otherwise I am forced to read through the code to see if the
> author got the reference counting logic correct."
>
> It costs _nothing_ to use it, and let's everyone know you got the logic
> correct.
>
> So don't feel it is a "abstraction", it's a helper for both the author
> (who doesn't have to get the atomic_t calls correct), and for everyone
> else who has to read the code.

kref abstraction is good.

Its current implementation seems straightforward, so I understand some devs 
think they can 'just use an atomic_t' themselves.

But using kref is better because some generic improvement could be done at 
kref level and you dont need to parse all kernel sources to change atomic_t to 
kref...

Example of improvement in kref_put() :

[PATCH] kref : Avoid an atomic operation in kref_put() when the last reference 
is dropped. On most platforms, atomic_read() is a plan read of the counter and 
involves no atomic at all.


Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
--- a/lib/kref.c	2006-01-30 06:11:04.000000000 +0100
+++ b/lib/kref.c	2006-01-30 06:13:32.000000000 +0100
@@ -52,7 +52,12 @@
 	WARN_ON(release == NULL);
 	WARN_ON(release == (void (*)(struct kref *))kfree);
 
-	if (atomic_dec_and_test(&kref->refcount)) {
+	/*
+	 * if current count is one, we are the last user and can release object
+	 * right now, avoiding an atomic operation on 'refcount'
+	 */
+	if ((atomic_read(&kref->refcount) == 1) ||
+	    (atomic_dec_and_test(&kref->refcount))) {
 		release(kref);
 		return 1;
 	}