linux-kernel - Re: [Xen-devel] [PATCH v5 2/6] xen/pciback: Don't deadlock when unbinding.

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Message-ID: <53E0A391.8090208@citrix.com>
Date:	Tue, 5 Aug 2014 10:27:45 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	<gregkh@...uxfoundation.org>, <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
	<xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] [PATCH v5 2/6] xen/pciback: Don't deadlock when unbinding.

On 04/08/14 19:42, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 28, 2014 at 02:06:59PM +0100, David Vrabel wrote:
>> On 14/07/14 17:18, Konrad Rzeszutek Wilk wrote:
>>> As commit 0a9fd0152929db372ff61b0d6c280fdd34ae8bdb
>>> 'xen/pciback: Document the entry points for 'pcistub_put_pci_dev''
>>> explained there are four entry points in this function.
>>> Two of them are when the user fiddles in the SysFS to
>>> unbind a device which might be in use by a guest or not.
>>>
>>> Both 'unbind' states will cause a deadlock as the the PCI lock has
>>> already been taken, which then pci_device_reset tries to take.
>>>
>>> We can simplify this by requiring that all callers of
>>> pcistub_put_pci_dev MUST hold the device lock. And then
>>> we can just call the lockless version of pci_device_reset.
>>>
>>> To make it even simpler we will modify xen_pcibk_release_pci_dev
>>> to quality whether it should take a lock or not - as it ends
>>> up calling xen_pcibk_release_pci_dev and needs to hold the lock.
>>>
>>> CC: stable@...r.kernel.org
>>
>> This deadlock is for a rather specific and uncommon use case (manually
>> unbinding a PCI while it is passed-through). Is this critical enough to
>> warrant a stable backport?
> 
> We seem to trip over it frequently when rebooting a server.
> 
> That is the VF's end up being unbinded while the guests
> are being shutdown. And depending on the timing we end up in a deadlock.

Ok.  I'll add the stable tag.

David
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