5.7.4.2. Mutexes

• semaphore vs mutex

• semaphores not obsolete after all

• mutexes 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:

#include <linux/mutex.h>

DEFINE_MUTEX(name);

at runtime:

mutex_init(struct mutex *lock);

locking:

void mutex_lock(struct mutex *lock);
int mutex_lock_interruptible(struct mutex *lock);
int mutex_trylock(struct mutex *lock);

unlocking:

void mutex_unlock(struct mutex *lock);

testing state:

int mutex_is_locked(struct mutex *lock);