Linux驱动之互斥锁

互斥锁是为了替代信号量，修改于2.6.10-2.6.22。参考：http://blog.chinaunix.net/uid-26023319-id-2980285.html

描述：

/*
* Simple, straightforward mutexes with strict semantics:
*
* - only one task can hold the mutex at a time
* - only the owner can unlock the mutex
* - multiple unlocks are not permitted
* - recursive locking is not permitted
* - a mutex object must be initialized via the API
* - a mutex object must not be initialized via memset or copying
* - task may not exit with mutex held
* - memory areas where held locks reside must not be freed
* - held mutexes must not be reinitialized
* - mutexes may not be used in hardware or software interrupt
*
contexts such as tasklets and timers
*
* These semantics are fully enforced when DEBUG_MUTEXES is
* enabled. Furthermore, besides enforcing the above rules, the mutex
* debugging code also implements a number of additional features
* that make lock debugging easier and faster:
*
* - uses symbolic names of mutexes, whenever they are printed in debug output
* - point-of-acquire tracking, symbolic lookup of function names
* - list of all locks held in the system, printout of them
* - owner tracking
* - detects self-recursing locks and prints out all relevant info
* - detects multi-task circular deadlocks and prints out all affected
*
locks and tasks (and only those tasks)
*/
struct mutex {
/* 1: unlocked, 0: locked, negative: locked, possible waiters */
atomic_t
count;
spinlock_t
wait_lock;
struct list_head	wait_list;
#if defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_SMP)
struct thread_info	*owner;
#endif
#ifdef CONFIG_DEBUG_MUTEXES
const char
*name;
void
*magic;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map	dep_map;
#endif
};

注意：不能嵌套锁，不能使用在tasklet或则timer这样的中断上下文中。

声明：

#define __MUTEX_INITIALIZER(lockname) 
{ .count = ATOMIC_INIT(1) 
, .wait_lock = __SPIN_LOCK_UNLOCKED(lockname.wait_lock) 
, .wait_list = LIST_HEAD_INIT(lockname.wait_list) 
__DEBUG_MUTEX_INITIALIZER(lockname) 
__DEP_MAP_MUTEX_INITIALIZER(lockname) }
#define DEFINE_MUTEX(mutexname) 
struct mutex mutexname = __MUTEX_INITIALIZER(mutexname)
extern void __mutex_init(struct mutex *lock, const char *name,
struct lock_class_key *key);

获取锁：

/***
* mutex_lock - acquire the mutex
* @lock: the mutex to be acquired
*
* Lock the mutex exclusively for this task. If the mutex is not
* available right now, it will sleep until it can get it.
*
* The mutex must later on be released by the same task that
* acquired it. Recursive locking is not allowed. The task
* may not exit without first unlocking the mutex. Also, kernel
* memory where the mutex resides mutex must not be freed with
* the mutex still locked. The mutex must first be initialized
* (or statically defined) before it can be locked. memset()-ing
* the mutex to 0 is not allowed.
*
* ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
*
checks that will enforce the restrictions and will also do
*
deadlock debugging. )
*
* This function is similar to (but not equivalent to) down().
*/
void __sched mutex_lock(struct mutex *lock)
{
might_sleep();
/*
* The locking fastpath is the 1->0 transition from
* 'unlocked' into 'locked' state.
*/
__mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
mutex_set_owner(lock);
}

/***
* mutex_unlock - release the mutex
* @lock: the mutex to be released
*
* Unlock a mutex that has been locked by this task previously.
*
* This function must not be used in interrupt context. Unlocking
* of a not locked mutex is not allowed.
*
* This function is similar to (but not equivalent to) up().
*/
void __sched mutex_unlock(struct mutex *lock)
{
/*
* The unlocking fastpath is the 0->1 transition from 'locked'
* into 'unlocked' state:
*/
#ifndef CONFIG_DEBUG_MUTEXES
/*
* When debugging is enabled we must not clear the owner before time,
* the slow path will always be taken, and that clears the owner field
* after verifying that it was indeed current.
*/
mutex_clear_owner(lock);
#endif
__mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
}

mutex_lock_interruptible 

mutex_lock_killable

mutex_lock_nested

mutex_lock_interruptible_nested

mutex_lock_killable_nested

mutex_trylock

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