Messages in this thread Patch in this message |  | | Date | Mon, 30 Jan 2006 06:19:35 +0100 | | From | Eric Dumazet <> | | | Subject | Re: [PATCH 1/5] pid: Implement task references. |
Greg KH a écrit :
> On Sun, Jan 29, 2006 at 02:58:51PM -0700, Eric W. Biederman wrote:
>> Greg KH <greg@kroah.com> 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@cosmosbay.com>
--- 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;
} |  |