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From: Oliver Neukum <oliver@neukum.org>
To: Duncan Sands <baldrick@free.fr>, Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [linux-usb-devel] Re: [OOPS,  usbcore, releaseintf] 2.6.0-test10-mm1
Date: Thu, 11 Dec 2003 11:19:00 +0100
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Cc: David Brownell <david-b@pacbell.net>, Vince <fuzzy77@free.fr>,
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References: <Pine.LNX.4.44L0.0312081754480.2034-100000@ida.rowland.org> <200312101758.02334.oliver@neukum.org> <200312111045.11275.baldrick@free.fr>
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Am Donnerstag, 11. Dezember 2003 10:45 schrieb Duncan Sands:
> > > > > __usb_set_configuration - lockless version
> > > > > usb_set_configuration - locked version
> > > >
> > > > Partially done.
> > > > That's what the _physical version of usb_reset_device() is about.
> > >
> > > Unfortunately, usb_physical_reset_device calls usb_set_configuration
> > > which takes dev->serialize.
> >
> > That is bad, but the solution is obvious.
> > All such operations need a _physical version.
> > At first sight this may look less elegant than some lock dropping schemes,
> > but it is a solution that produces obviously correct code paths with
> > respect to locking.
> 
> Hi Oliver, I agree, except that there are several layers of locking: dev->serialize
> but also the bus rwsem.  So does "physical" mean no subsys.rwsem or no
> dev->serialize or both?

"physical" means no locking at all. It's the caller's responsibility.

> int usb_reset_device(struct usb_device *udev)
> {
>         struct device *gdev = &udev->dev;
>         int r;
> 
>         down_read(&gdev->bus->subsys.rwsem);
>         r = usb_physical_reset_device(udev);
>         up_read(&gdev->bus->subsys.rwsem);
> 
>         return r;
> }

That's what the core cares about. No probe() while a reset is in
progress. Taking the semaphore ensures that.

	Regards
		Oliver