Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 31 Aug 2018 23:08:33 -0700 (PDT)
Message-Id: <20180831.230833.1182096350264602698.davem@davemloft.net>
To:     decui@microsoft.com
Cc:     kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        netdev@vger.kernel.org, jopoulso@microsoft.com, olaf@aepfle.de,
        jasowang@redhat.com, linux-kernel@vger.kernel.org,
        marcelo.cerri@canonical.com, apw@canonical.com,
        devel@linuxdriverproject.org, vkuznets@redhat.com
Subject: Re: [PATCH v2] hv_netvsc: Fix a deadlock by getting rtnl lock
 earlier in netvsc_probe()
From:   David Miller <davem@davemloft.net>
In-Reply-To: <PU1P153MB0169F05DF2A05DC39B4BF360BF080@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM>
References: <PU1P153MB0169F05DF2A05DC39B4BF360BF080@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM>
Mime-Version: 1.0
Content-Type: Text/Plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Dexuan Cui <decui@microsoft.com>
Date: Thu, 30 Aug 2018 05:42:13 +0000

> 
> This patch fixes the race between netvsc_probe() and
> rndis_set_subchannel(), which can cause a deadlock.
> 
> These are the related 3 paths which show the deadlock:
 ...
> Before path #1 finishes, path #2 can start to run, because just before
> the "bus_probe_device(dev);" in device_add() in path #1, there is a line
> "object_uevent(&dev->kobj, KOBJ_ADD);", so systemd-udevd can
> immediately try to load hv_netvsc and hence path #2 can start to run.
> 
> Next, path #2 offloads the subchannal's initialization to a workqueue,
> i.e. path #3, so we can end up in a deadlock situation like this:
> 
> Path #2 gets the device lock, and is trying to get the rtnl lock;
> Path #3 gets the rtnl lock and is waiting for all the subchannel messages
> to be processed;
> Path #1 is trying to get the device lock, but since #2 is not releasing
> the device lock, path #1 has to sleep; since the VMBus messages are
> processed one by one, this means the sub-channel messages can't be
> procedded, so #3 has to sleep with the rtnl lock held, and finally #2
> has to sleep... Now all the 3 paths are sleeping and we hit the deadlock.
> 
> With the patch, we can make sure #2 gets both the device lock and the
> rtnl lock together, gets its job done, and releases the locks, so #1
> and #3 will not be blocked for ever.
> 
> Fixes: 8195b1396ec8 ("hv_netvsc: fix deadlock on hotplug")
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

Applied and queued up for -stable.