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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