Messages in this thread | | | Date | Sat, 13 Dec 2003 04:15:01 +0100 | From | Manfred Spraul <> | Subject | Re: [DOCUMENTATION] Revised Unreliable Kernel Locking Guide |
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Hi Rusty,
From Chapter 4.1:
> |spin_lock_irqsave()| (include/linux/spinlock.h) is a variant which > saves whether interrupts were on or off in a flags word, which is > passed to |spin_unlock_irqrestore()|. This means that the same code > can be used inside an hard irq handler (where interrupts are already > off) and in softirqs (where the irq disabling is required).
Interrupts are typically on within the hard irq handler. spin_lock() is usually ok because an interrupt handler is never reentered. Thus if a lock is only accessed from a single irq handler, then spin_lock() is the faster approach. That's why many nic drivers use spin_lock instead of spin_lock_irqsave() in their irq handlers. OTHO: if a driver lock is a global resource that is used from different irqs, then it must either use _irqsave(), or set SA_INTERRUPT. Examples: rtc_lock relies on SA_INTERRUPT: it's touched from the rtc irq and the timer irq path, and both rtc and timer set SA_INTERRUPT. I assume ide relies on _irqsave(), but the code is too difficult to follow.
Btw, perhaps you could add the 2nd synchronization approach for interrupts: if it's an extremely rare event, then no lock at all in the irq handler (no reentrancy guaranteed by the kernel), and spin_lock+disable_irq() in the softirq/tasklet. My network drivers use that to synchronize packet rx with close.
-- Manfred
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