linux-kernel - Re: [Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to remotely connect USB devices over IP.

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Date:	Fri, 3 Jul 2015 10:04:54 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Krzysztof Opasiak <k.opasiak@...sung.com>
cc:	Jeremy White <jwhite@...eweavers.com>,
	Oliver Neukum <oneukum@...e.com>,
	Hans de Goede <hdegoede@...hat.com>,
	"Daniel P. Berrange" <berrange@...hat.com>,
	<spice-devel@...ts.freedesktop.org>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to
 remotely connect USB devices over IP.

On Fri, 3 Jul 2015, Krzysztof Opasiak wrote:

> > 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?

Yes indeed.

> 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.

That is good advice.  :-)

Alan Stern

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