Interrupts, Spin Locks, and Preemption

Interrupts

• Process context vs. Interrupt context

  • system calls run in process context – can sleep

  • interrupt handlers run in interrupt context – cannot sleep

• Interrupt handler

  • single interrupt will not nest, so hander need not be reentrant
    • but handler can be interrupted by a different interrupt
  • only time-critical stuff in handlers
    • push rest to bottom half

  • all handlers share one interrupt stack per processor

Spin Locks

• spin_lock() / spin_unlock()

  • must not lose CPU while holding a spin lock
    • other threads will wait for the lock for a long time
  • spin_lock() prevents kernel preemption by ++preempt_count
    • in uniprocessor, that’s all spin_lock() does
  • must NOT call any function that can potentially sleep
    • ex) kmalloc, copy_from_user
  • hardware interrupt is ok unless the interrupt handler may try to lock this spin lock
    • spin lock not recursive: same thread locking twice will deadlock

  • keep the critical section as small as possible

• spin_lock_irqsave() / spin_unlock_irqrestore()

  • disable all interrupts on local CPU, lock, unlock, restore interrupts to how it was before

  • need to use this version if the lock is something that an interrupt handler may try to acquire

  • no need to worry about interrupts on other CPUs – spin lock will work normally

  • again, no need to spin in uniproc – just ++preempt_count & disable irq

• spin_lock_irq() / spin_unlock_irq()

  • disable & enable irq assuming it was disabled to begin with

  • should not be used in most cases

Preemption

If TIF_NEED_RESCHED is set, preemption occurs by calling schedule() in the following cases:

1. Returning to user space:

   • from a system call

   • from an interrupt handler

2. Returning to kernel from an interrupt handler, only if preempt_count is zero

3. preempt_count just became zero – right after spin_unlock(), for example

4. Thread running in kernel mode calls schedule() itself – blocking syscall, for example

Last updated: 2016–03–31