LddMutex < Training2

%SECTION0{name=LddMutex}% Mutexes

   * [[http://koti.mbnet.fi/niclasw/MutexSemaphore.html][semaphore vs mutex]]

   * [[http://lwn.net/Articles/273731/][semaphores not obsolete after all]]

   * mutexes [[http://lwn.net/Articles/167034/][lwn article]]

   * binary semaphore: locked or unlocked

   * used for protecting critical paths

   * restrictions:
      * one task can hold mutex at a time
      * only task which locks can unlock again
      * no recursive/multiple locking/unlocking
      * can't be used in interrupt context (hard or soft)

   * Advantages (over semaphores):
      * Plattform independant
      * faster than semphores (smaller!)
      * code may sleep
      * =DEBUG_MUTEXES= allows effective debugging

   * Disadvantages
      * only binary (no counting)


---++API: Kernel Mutexes

_initalize:_

<verbatim>
#include <linux/mutex.h>
</verbatim>

<verbatim>
DEFINE_MUTEX(name);
</verbatim>

_at runtime:_
<verbatim>
mutex_init(struct mutex *lock);
</verbatim>


_locking:_

<verbatim>
void mutex_lock(struct mutex *lock);
int mutex_lock_interruptible(struct mutex *lock);
int mutex_trylock(struct mutex *lock);
</verbatim>

_unlocking:_
<verbatim>
void mutex_unlock(struct mutex *lock);
</verbatim>

_testing state:_
<verbatim>
int mutex_is_locked(struct mutex *lock);
</verbatim>
5.7.4.1. Atomic operations		1. Denx Training Topics		5.7.4.3. Spinlocks
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