On Wed, Feb 12, 2014 at 07:35:24PM +0200, Michael S. Tsirkin wrote:

On Wed, Feb 12, 2014 at 08:56:30AM -0800, Greg Kroah-Hartman wrote:

quoted

On Wed, Feb 12, 2014 at 06:38:21PM +0200, Michael S. Tsirkin wrote:

quoted

It is sometimes useful to get the value of the reference count after
decrement.
For example, vhost wants to execute some periodic cleanup operations
once number of references drops below a specific value, before it
reaches zero (for efficiency).

You should never care about what the value of the kref is, if you are
using it correctly :)

So I really don't want to add this function, as I'm sure people will use
it incorrectly.  You should only care if the reference drops to 0, if
not, then your usage doesn't really fit into the "kref" model, and so,
just use an atomic variable.

This happens when you have code that keeps
reference itself implicitly or explicitly.

	foo(struct kref *k, int bar) {

	sub = kref_sub(k)

	if (sub == 1)
		FOO(k, bar) /* Here I am the only one
			       with a reference */

Why do you care if you are the only one with a reference?

If you do, then just don't grab that reference and do the work in the
cleanup callback :)

	}

	kref_get(k)
	foo(k, bar);
	....
	kref_put(k)

Why not do FOO in destructor you ask?
Absolutely but this will be called much later.

Maybe you will reconsider if I document this
as the only legal use?

No one reads documentation :(

quoted

I really want to know why it matters for "efficiency" that you know this
number.  How does that help anything, as the number could then go up
later on, and the work you did at a "lower" number is obsolete, right?

thanks,

greg k-h

The issue is that if number dropped to 1, this means
we must do the cleanup work since there are
no outstanding buffers, (last user is ourselves)
if we do not cleanup,
guest will hang waiting for us.

This doesn't make sense, nor does it sound like a use for a kref (or you
are using it wrong.)

But it never drops to 0 since we have our own reference
in the device.

Then don't do that.

If it goes up again this means we didn't have
to do cleanup, but an alternative is doing
it all the time and that is slow.

Then just cleanup when it hits 0, like the rest of the world does.

Yes I can rework vhost to open-code this kref use, it's
no big deal.
Alternatively since most of the use does match kref
model, maybe __kref_sub_return with disclaimers
that you must know what you are doing?

No, no one reads documentation, sorry.  Either fix your use of kref
(i.e. don't care about the count), or do something else, as you don't
want a kref, but rather, an atomic count of what is going on and "1"
means something "special" to you.

sorry,

greg k-h