LddOtherLockingTechniques < Training2

%SECTION0{name=LddOtherLockingTechniques}% Other locking techniques

*reader/writer semaphores and spinlocks:*

   * situation: many reader, few writers
   * allow multiple readers to access the data simultaneously, but only one writer


*completions:*
   * situation: start something and be notified once it completes
   * previously done with semaphores (deprecated in favor of mutexes)


*RCU: read-copy-update:*

   * [[http://lwn.net/Articles/262464/][detailed LWN article]]
   * advanced, high performance lock-free mechanism
   * situation: many reads, few writes
   * mechanism (extremely simplified):
      * references to protected data must be held only by atomic code
      * when protected data must be changed, writer makes copy, changes it and then updates pointer
      * problem: code on other CPUs might hold reference to old version
      * solution: because code holding rcu reference must be atomic, old ref must be gone after reschedule of all CPUs

*PI mutexes:*
   * situation: low priority threads holding a lock shared with an important high priority thread is preempted by a CPU intensive medium priority thread: low priority thread can't finish and free-lock so high priority thread starves.
   * mechanism: a thread holding a pi-mutex inherits priority of highest waiter
   * interesting: Mars Pathfinder experienced this problem. [[http://research.microsoft.com/research/os/mbj/Mars_Pathfinder/Mars_Pathfinder.html][read here]]

*seqlocks:*

   * provide fast lockless access to shared resource
   * situation: resource is small, simple and frequently read. write access is rare but must be fast
   * mechanism: readers have free access, but must check for collisions with writers (and then retry)
5.7.4.3. Spinlocks		1. Denx Training Topics		5.7.5. Allocating Memory
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