RT-friendly IRQ management in USB

Steven:

At OSL you mentioned that local_irq_{save,disable} is unfriendly for RT
and spin_lock_irq[save] is greatly preferred.  As it happens, the USB
core contains several instances of this code pattern:

	local_irq_save(flags);
	spin_lock(&some_lock);
	...
	spin_unlock(&some_lock);
	usb_hcd_giveback_urb(hcd, urb);
	local_irq_restore(flags);

The reason for doing this is historical; usb_hcd_giveback_urb() is
documented as running with local IRQs disabled, and many drivers depend
on that.  For example, their callback routines invoked by
usb_hcd_giveback_urb do spin_lock() instead of spin_lock_irqsave().

So what's the best way to do this?  Should we do:

	spin_lock_irqsave(&some_lock, flags);
	...
	spin_unlock(&some_lock);
	usb_hcd_giveback_urb(hcd, urb);
	local_irq_restore(flags);

or is there a better approach?

Also, the USB host controller drivers contain this code pattern:

	spin_lock_irq(&some_lock);
	...
	spin_unlock(&some_lock);
	usb_hcd_giveback_urb(hcd, urb);
	spin_lock(&some_lock);
	...
	spin_unlock_irq(&some_lock);

Again, this is because usb_hcd_giveback_urb() must run with interrupts 
disabled.  Is this pattern okay?

Alan Stern


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