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SubjectRe: interrupt - spin lock question
On Sunday 03 March 2002 09:18 pm, wrote:
> I have a question related to spin locking on UP systems.Before that I would
> like to point out my understanding of the background stuff
> 1. spinlocks shud be used in intr handlers

It should be used in the interrupt handler, if you need to prevent any race
conditions with other interrupt/non-interrupt context code that may be
executing on some other CPU on an SMP system. Thus spinlocks need to be held
for as short a duration as possible. You would need to use the
spin_lock_irqsave/spin_unlock_irqrestore variant pair to prevent your
interrupt handler from running on the same processor while holding the lock.
This may be needed if the interrupt handler may try to acquire the same lock
thus causing a deadlock.

> 2. interrupts can preempt kernel code
> 3. spinlocks are turned to empty when kernel is compiled without SMP
> support.
> If a particular driver is running( not the intr handler part) and at this
> time an interrupt occurs. The handler has to be invoked now. Won't the
> preemption cause race conditions/inconsistencies? Is any other mechanism
> used? Pl correct me if I have not understood any part of this correctly

On a UP kernel the spin_lock_irqsave/spin_unlock_irqrestore pair expand to
save_flags(flag); cli()/restore_flags(flag).

The masking of interrupts on the processor between spin_lock_irqsave and
spin_unlock_irqrestore pair prevent the user context code from being
preempted by the interrupt handler.

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