netdev - Re: [PATCH net 1/3] kref: add kref_sub

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Message-Id: <20140212.190637.328045386111912135.davem@davemloft.net>
Date:	Wed, 12 Feb 2014 19:06:37 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	gregkh@...uxfoundation.org
Cc:	mst@...hat.com, linux-kernel@...r.kernel.org, jasowang@...hat.com,
	virtio-dev@...ts.oasis-open.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	qinchuanyu@...wei.com, joern@...fs.org, anatol.pomozov@...il.com,
	nab@...ux-iscsi.org
Subject: Re: [PATCH net 1/3] kref: add kref_sub_return

From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Date: Wed, 12 Feb 2014 08:56:30 -0800

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

It isn't being used to determine when to destroy things.

They use it to as a heuristic of when to trigger polling.

Each ubuf attached gets a kref to the higher level virtio_net buffer
holding object, they want to trigger polling when that reference drops
to 1 or lower.

Right now they are reading the atomic refcount directly, which
I think is much worse than this helper.
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