LddSpinlocks < Training2

%SECTION0{name=LddSpinlocks}% Spinlocks

   * fast, busy waiting mutual exclusion mechanism

   * Mechanism:
      * Atomic variable is initalized to 1: available
      * =spin_lock()= atomically decrements value and checks if equals 0
         * if yes: lock obtained
         * if no:  no lock, spin in tight loop until value is 1
      * =spin_unlock()= sets value back to 1

   * must only be used in code that cannot sleep!
      * no userspace memory access
      * no memory allocation (except =GFP_ATOMIC=)
  
   * three flavors
      * =spin_lock()=, =spin_unlock()=
         * on UP, no preemption: optimized out
         * on UP with preemption: disable preemption
         * protects against concurrency with "regular" kernel code
      * =spin_lock_irqsave=, =spin_unlock_irqrestore=, =spin_lock_irq=, =spin_unlock_irq=
         * disables interrupts on local CPU
         * protects against concurrency with interrupt handlers
      * =spin_lock_bh=, =spin_unlock_bh=
         * disables soft interrupt on local CPU
         * protects against concurrency with bottom halves, tasklets, softirqs, ...

   * Advantages:
      * fast and simple
  
   * Disadvantages:
      * code must be atomic: can never sleep!
      * unfair! solution: [[http://lwn.net/Articles/267968/][ticket spinlocks (merged 2.6.25)]]
5.7.4.2. Mutexes		1. Denx Training Topics		5.7.4.4. Other locking techniques
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