Date: Wed, 12 Feb 2014 19:06:37 -0500 (EST)
Message-Id: <20140212.190637.328045386111912135.davem@davemloft.net>
To: gregkh@linuxfoundation.org
Cc: mst@redhat.com, linux-kernel@vger.kernel.org, jasowang@redhat.com,
        virtio-dev@lists.oasis-open.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        qinchuanyu@huawei.com, joern@logfs.org, anatol.pomozov@gmail.com,
        nab@linux-iscsi.org
Subject: Re: [PATCH net 1/3] kref: add kref_sub_return
From: David Miller <davem@davemloft.net>
In-Reply-To: <20140212165630.GA22991@kroah.com>
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	<1392222846-26699-2-git-send-email-mst@redhat.com>
	<20140212165630.GA22991@kroah.com>
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From: Greg Kroah-Hartman <gregkh@linuxfoundation.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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