Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: when spin_lock_irq (as opposed to spin_lock_irqsave) is appropriate? | Date | Mon, 13 Oct 2008 00:21:37 +0200 |
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On Sunday 12 October 2008, Andrey Borzenkov wrote: > This leaves me with a question - how can I know whether interrupts may > (not) be disabled at particular point? In particular, is it safe to > assume that any place marked at "code may sleep" has interrupts enabled?
Yes, that is safe. The only times you know that interrupts are disabled are:
1. If you have disabled interrupts yourself using local_irq_{disable,save} or spin_lock_irq{,save}.
2. If you get called from an interface that is documented to have interrupts disabled. The only common example of this is the interrupt handler function you register with request_irq().
In all other cases, interrupts are disabled, though in some places you may not sleep, e.g. because of spin_lock(), preempt_disable() or softirq context (timer, tasklet, ...). The question of whether you may sleep or not is irrelevant to whether or not you can use spin_lock_irq.
The rules are:
* If you know that interrupts are disabled, use spin_lock(). * If you know that interrupts are enabled and you might race against an interrupt handler, use spin_lock_irq(). * If you cannot race against a hard interrupt handler, but can race against a softirq, use spin_lock_bh(). * If you cannot race against either hardirq or softirq context but cannot sleep, use spin_lock(). * If you can sleep in all places that take the spinlock, replace the spinlock with a mutex. * If you cannot tell whether interrupts are enabled or disabled, but you can race against a hardirq, use spin_lock_irqsave.
Some people interpret the last rule as "If I can't be bothered to find out who is calling me, use spin_lock_irqsave", but I much prefer to be explicit (besides efficient) to give the reader a better indication of what the lock actually does.
Arnd <><
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