Message-id: <55964CFF.1090803@samsung.com>
Date: Fri, 03 Jul 2015 10:51:11 +0200
From: Krzysztof Opasiak <k.opasiak@samsung.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101
 Thunderbird/31.4.0
MIME-version: 1.0
To: Alan Stern <stern@rowland.harvard.edu>,
        Jeremy White <jwhite@codeweavers.com>
Cc: Oliver Neukum <oneukum@suse.com>, Hans de Goede <hdegoede@redhat.com>,
        "Daniel P. Berrange" <berrange@redhat.com>,
        spice-devel@lists.freedesktop.org, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to
 remotely connect USB devices over IP.
References: <Pine.LNX.4.44L0.1507021615250.1320-100000@iolanthe.rowland.org>
In-reply-to: <Pine.LNX.4.44L0.1507021615250.1320-100000@iolanthe.rowland.org>
Content-type: text/plain; charset=windows-1252; format=flowed
Content-transfer-encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2556
Lines: 56


On 07/02/2015 10:20 PM, Alan Stern wrote:
> On Thu, 2 Jul 2015, Jeremy White wrote:
>
>>> Oliver is talking about the danger of having part of the communication
>>> path for a block device run through userspace.
>>>
>>> Imagine a situation where the client uses a USB storage device provided
>>> by the server as a swap device.  And suppose a userspace daemon on the
>>> client has to process USB packets as they pass between the client and
>>> the server.  If the daemon is idle for some time, parts of its address
>>> space may get stored in the swap area on the server and paged out.
>>>
>>> Now consider what happens when those parts of memory need to be paged
>>> back in.  The client submits a request to read from the swap area.
>>> The request is transformed into USB packets and sent through the
>>> userspace daemon for transmission to the server.  But the daemon can't
>>> process the packets because it is waiting for its missing parts to be
>>> paged back!  Result: deadlock.
>>
>> Right.  I followed that.  Oliver also asserted that he believed that the
>> current usbip implementation has this flaw; I do not follow that.  The
>> concept is that the usbip device driver virtualizes the device behavior;
>> isolating the running kernel from the vagaries of the network transport.
>>   All proposed usbredir implementations, even if they move the network
>> transport to user space, would retain that behavior.
>
> The point is that a device driver like usbip _cannot_ isolate the
> running kernel from the vagaries of the network transport if part of
> that transport occurs in userspace.
>
> If any part of the transport passes through userspace, you can end up
> in a situation like what I outlined above, where a message can't be
> transported until after its reply has been received.  There's no way
> for a device driver to prevent a deadlock when this occurs, no matter
> what it virtualizes.
>

Doesn't we have the same problem with functionfs/gadgetfs and dummy_hcd? 
Or with fuse?

It's a very generic problem for all "virtualized devices" and it is 
known for quite a long time. This is why many tutorials about swap warns 
that swap should be set up only on real block devices which are fully 
served in kernel.

-- 
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/