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From: Hans Petter Selasky <hselasky@freebsd.org>
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Date: Sun, 28 Aug 2011 11:27:44 +0200
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Cc: Andriy Gapon <avg@freebsd.org>
Subject: Re: skipping locks, mutex_owned, usb
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On Sunday 28 August 2011 11:25:51 Andriy Gapon wrote:
> on 23/08/2011 15:09 Andriy Gapon said the following:
> > This "XXX cludge" [sic] pattern is scattered around a few functions in
> > the ukbd code and perhaps other usb code:
> > func()
> > {
> > 
> > 	if (!mtx_owned(&Giant)) {
> > 	
> > 		mtx_lock(&Giant);
> > 		
> >                 func();
> >                 mtx_unlock(&Giant);
> > 		
> > 		return;
> > 	
> > 	}
> > 	
> > 	// etc ...
> > 
> > }
> 
> Ohhh, nothing seems simple with the USB code:
> 
> /* make sure that the BUS mutex is not locked */
> drop_bus = 0;
> while (mtx_owned(&xroot->udev->bus->bus_mtx)) {
>         mtx_unlock(&xroot->udev->bus->bus_mtx);
>         drop_bus++;
> }
> 
> /* make sure that the transfer mutex is not locked */
> drop_xfer = 0;
> while (mtx_owned(xroot->xfer_mtx)) {
>         mtx_unlock(xroot->xfer_mtx);
>         drop_xfer++;
> }
> 
> So many unconventional tricks.

Similar code is used in the DROP_GIANT and PICKUP_GIANT macros. You might want 
to check all references to mtx_owned() in the kernel, and make a set of 
exceptions for post-panic code execution.

--HPS