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From: Hans Petter Selasky <hselasky@c2i.net>
To: John Baldwin <jhb@freebsd.org>
Subject: Re: svn commit: r246614 - head/sys/dev/usb/wlan
Date: Tue, 12 Feb 2013 08:43:32 +0100
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On Monday 11 February 2013 17:33:02 John Baldwin wrote:
> On Sunday, February 10, 2013 5:36:17 am Hans Petter Selasky wrote:
> > Author: hselasky
> > Date: Sun Feb 10 10:36:16 2013
> > New Revision: 246614
> > URL: http://svnweb.freebsd.org/changeset/base/246614
> > 
> > Log:
> >   - Streamline detach logic in wlan drivers, so that
> >   
> >     freed memory cannot be used during detach.
> >   
> >   - Remove all panic() calls from the urtw driver because
> >   
> >     panic() is not appropriate here.
> >   
> >   - Remove redundant checks for device detached in
> >   
> >     device detach callbacks.
> >   
> >   - Use DEVMETHOD_END to mark end of device methods.
> 
> Using a detached flag to bail from ioctl generally means you are doing
> things wrong in detach.  The correct solution is to always detach your
> ifnet first, then start tearing down other state.  In general with device
> detach routines the first order of business is removing external
> references such as character devices, ifnets, etc. and only start shutting
> down the hardware and releasing state once those steps have completed.

Hi,

What I can do to solve the problem is to lock a mutex while detaching. Is the 
ifnet detach routine non-blocking?

Why do I say that? It is because we are in a chicken-egg situation. USB is 
feeding data into ifnet and ifnet is feeding data into USB. Each stack is 
running under its own lock. The current approach is:

1) Stop data traffic in both directions
2) Make sure no more init happens (ioctl fix)
3) Free USB and IFNET.

I see currently no way I can atomically stop all traffic and prevent futher 
init at the same time, while holding a single mutex. Do you?

--HPS