Let's quickly summarize spinlocks:
- Simplest, lowest overhead: Use the non-irq spinlock primitives, spin_lock()/spin_unlock(), when protecting critical sections in the process context (there's either no interrupts to deal with or there are interrupts, but we do not race with them at all; in effect, use this when interrupts don't come into play or don't matter).
- Medium overhead: Use the irq-disabling (as well as kernel preemption disabling) versions, spin_lock_irq() / spin_unlock_irq(), when interrupts are in play and do matter (the process and interrupt contexts can "race"; that is, they share global data).
- Strongest (relatively), high overhead: This is the safest way to use a spinlock. It does the same as the medium overhead, except it performs a save-and-restore on the interrupt mask via the spin_lock_irqsave() / spin_unlock_irqrestore() pair, so as to guarantee that the...
