Re: [PATCH net 1/3] kref: add kref_sub_return

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Wed, 12 Feb 2014 23:05:06 -0500 (EST)

> From: Greg KH <gregkh@linuxfoundation.org>
> Date: Wed, 12 Feb 2014 17:39:02 -0800
> 
>> Yes, that's horrible as well, but as was already pointed out in this
>> thread, you can't rely on that value to really be "1" after reading it
>> due to the way krefs work, what happened if someone else just grabbed
>> it?
>> 
>> If all they want is a "count" for when to start polling, then use a
>> separate atomic count, but don't abuse the kref interface for this, I
>> don't think that will work properly at all.
> 
> They want to know which thread of control decrements the count to "1"
> as buffers are released.
> 
> That seems entirely reasonable to me.
> 
> They could add another atomic counter for this, but that's rather
> silly since the kref already has an atomic they can use for this
> purpose.

If you still can't understand what they are trying to do, they want to
do something precisely when the number of pending buffers is dropped
to 1 or less.

They are using krefs to track how many buffers are attached at a given
moment.

The counter can re-increment after the decrement to 1 or less occurs,
they don't care.

But they want precisely the entity that drops it down to 1 or less to
perform that action.

Just reading the atomic value directly, they cannot do this.