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Date:   Thu, 31 Jan 2019 12:47:07 -0500
From:   Kimberly Brown <kimbrownkd@gmail.com>
To:     Michael Kelley <mikelley@microsoft.com>
Cc:     Sasha Levin <sashal@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Long Li <longli@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Drivers: hv: vmbus: Add mutex lock to channel show
 functions
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On Thu, Jan 31, 2019 at 04:45:35PM +0000, Michael Kelley wrote:
> From: Sasha Levin <sashal@kernel.org> Sent: Thursday, January 31, 2019 7:20 AM
> > 
> > I've queued this one for hyper-fixes, thanks all!
> > 
> 
> Actually, please hold off on queuing this one.  In a conversation I had yesterday with Kim, they had identified a deadlock.  Kim was going to be looking at some revisions to avoid the deadlock.
> 
> Kim -- please confirm.
> 
> Michael

This is correct. I need to send a v2 of this patch which addresses two
issues:

1) Check channel->state inside the mutex acquire/release in
vmbus_chan_attr_show().

2) Prevent a deadlock that can occur between the proposed mutex_lock()
call in the vmbus_chan_attr_show() function and the sysfs/kernfs functions.


I've identified two possible solutions to the deadlock:

1) Add a new mutex to the vmbus_channel struct which protects changes to
"channel->state". Use this new mutex in vmbus_chan_attr_show() instead of
"vmbus_connection.channel_mutex".

2) Use "vmbus_connection.channel_mutex" in vmbus_chan_attr_show() as
originally proposed, and acquire it with mutex_trylock(). If the mutex
cannot be acquired, return -EINVAL.


I'm leaning towards (2), using mutex_trylock(). "vmbus_connection.channel_mutex"
appears to be used only when a channel is being opened or closed, so
vmbus_chan_attr_show() should be able to acquire the mutex when the
channel is in use.

If anyone has suggestions, please let me know.

Thanks,
Kim