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

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Message-Id: <20140213.184812.213639748986818676.davem@davemloft.net>
Date:	Thu, 13 Feb 2014 18:48:12 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	mst@...hat.com
Cc:	linux-kernel@...r.kernel.org, qinchuanyu@...wei.com,
	kvm@...r.kernel.org, virtio-dev@...ts.oasis-open.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	jasowang@...hat.com
Subject: Re: [PATCH net v2] vhost: fix ref cnt checking deadlock

From: "Michael S. Tsirkin" <mst@...hat.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@...wei.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
> 
> This patch is needed for 3.14 and -stable.

Applied and queued up for -stable.
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