Re: [PATCH net v2] vhost: fix ref cnt checking deadlock

[David Miller <davem@davemloft.net>]

From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Thu, 13 Feb 2014 11:42:05 +0200

> vhost checked the counter within the refcnt before decrementing.  It
> really wanted to know that it is the one that has the last reference, as
> a way to batch freeing resources a bit more efficiently.
> 
> Note: we only let refcount go to 0 on device release.
> 
> This works well but we now access the ref counter twice so there's a
> race: all users might see a high count and decide to defer freeing
> resources.
> In the end no one initiates freeing resources until the last reference
> is gone (which is on VM shotdown so might happen after a looooong time).
> 
> Let's do what we probably should have done straight away:
> switch from kref to plain atomic, documenting the
> semantics, return the refcount value atomically after decrement,
> then use that to avoid the deadlock.
> 
> Reported-by: Qin Chuanyu <qinchuanyu@huawei.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 
> This patch is needed for 3.14 and -stable.

Applied and queued up for -stable.