netdev - Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

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Date:	Wed, 4 Nov 2009 15:17:36 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-mm@...ck.org, akpm@...ux-foundation.org
Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

On Wed, Nov 04, 2009 at 02:15:33PM +0100, Andi Kleen wrote:
> On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> > On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > > Fine?
> > > 
> > > I cannot say -- are there paths that could drop the device beforehand?
> > 
> > Do you mean drop the mm reference?
> 
> No the reference to the device, which owns the mm for you.

The device is created when file is open and destroyed
when file is closed. So I think the fs code handles the
reference counting for me: it won't call file cleanup
callback while some userspace process has the file open.
Right?

> > 
> > > (as in do you hold a reference to it?)
> > 
> > By design I think I always have a reference to mm before I use it.
> > 
> > This works like this:
> > ioctl SET_OWNER - calls get_task_mm, I think this gets a reference to mm
> > ioctl SET_BACKEND - checks that SET_OWNER was run, starts virtqueue
> > ioctl RESET_OWNER - stops virtqueues, drops the reference to mm
> > file close - stops virtqueues, if we still have it then drops mm
> > 
> > This is why I think I can call use_mm/unuse_mm while virtqueue is running,
> > safely.
> > Makes sense?
> 
> Do you protect against another thread doing RESET_OWNER in parallel while
> RESET_OWNER runs?

Yes, I have a mutex in the device for that. Same with SET_BACKEND.

> -Andi
> 
> -- 
> ak@...ux.intel.com -- Speaking for myself only.
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