Linux内核同步机制mutex

mutex锁概述

在linux内核中，互斥量mutex是一种保证CPU串行运行的睡眠锁机制。和spinlock类似，都是同一个时刻只有一个线程进入临界资源，不同的是，当无法获取锁的时候，spinlock原地自旋，而mutex则是选择挂起当前线程，进入阻塞状态。所以，mutex无法在中断上下文中使用。

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mutex锁使用注意事项

mutex一次只能有一个进程或线程持有该锁mutex只有它的拥有者可以释放该锁不能多次释放同一把锁不可以重复获取同一把锁，否则会造成死锁必须使用mutex提供的专用初始化函数初始化该锁不能重复初始化同一把锁不能使用memset或memcpy等内存处理函数初始化mutex锁线程退出时要释放自己持有的所有mutex锁不能用于设备中断或软中断上下文中

mutex锁结构体定义

owner：记录mutex的持有者wait_lock：spinlock自旋锁soq：MCS锁队列，用于支持mutex乐观自旋机制wait_list：当无法获取锁的时候挂起在此magic：用于debug调试dep_map：用于debug调试

struct mutex { atomic_long_t  owner; spinlock_t  wait_lock;#ifdef CONFIG_MUTEX_SPIN_ON_OWNER struct optimistic_spin_queue osq; /* Spinner MCS lock */#endif struct list_headwait_list;#ifdef CONFIG_DEBUG_MUTEXES void   *magic;#endif#ifdef CONFIG_DEBUG_LOCK_ALLOC struct lockdep_map dep_map;#endif};

mutex锁主要接口函数

mutex_init	初始化mutex对象
__mutex_init	mutex_init会调用此函数
DEFINE_MUTEX	静态定义并初始化一个mutex对象
__MUTEX_INITIALIZER	DEFINE_MUTEX会调用此函数
mutex_lock	获取mutex锁，失败进程进入D状态
mutex_lock_interruptible	获取mutex锁，失败进入S状态
mutex_trylock	尝试获取mutex锁，失败直接返回
mutex_unlock	释放mutex锁
mutex_is_locked	判断当前mutex锁的状态

获取锁流程分析

mutex_lock()函数调用might_sleep()函数判断锁的状态，调用__mutex_trylock_fast()函数尝试快速获取mutex锁，如果失败，则调用__mutex_lock_slowpath()函数获取mutex锁

void __sched mutex_lock(struct mutex *lock){ might_sleep(); if (!__mutex_trylock_fast(lock))  __mutex_lock_slowpath(lock);}

如果没有定义CONFIG_DEBUG_ATOMIC_SLEEP宏，might_sleep函数退化为 might_resched()函数。

# define might_sleep() \\ do { __might_sleep(__FILE__, __LINE__, 0); might_resched(); } while (0)# define sched_annotate_sleep() (current- >task_state_change = 0)#else  static inline void ___might_sleep(const char *file, int line,       int preempt_offset) { }  static inline void __might_sleep(const char *file, int line,       int preempt_offset) { }# define might_sleep() do { might_resched(); } while (0)# define sched_annotate_sleep() do { } while (0)

在配置了抢占式内核或者非抢占式内核的情况下，might_resched()函数最终都是空函数。如果配置了主动抢占式内核CONFIG_PREEMPT_VOLUNTARY，则might_resched()函数会调用 _cond_resched()函数来主动触发一次抢占。

#ifdef CONFIG_PREEMPT_VOLUNTARYextern int _cond_resched(void);# define might_resched() _cond_resched()#else# define might_resched() do { } while (0)#endif#ifndef CONFIG_PREEMPTextern int _cond_resched(void);#elsestatic inline int _cond_resched(void) { return 0; }#endif

——cond_resched()函数调用should_resched()函数判断抢占计数器是否为0，如果抢占计数器为0并且设置了重新调度标记则调用preempt_schedule_common()函数进行抢占式调度

#ifndef CONFIG_PREEMPTint __sched _cond_resched(void){ if (should_resched(0)) {  preempt_schedule_common();  return 1; } return 0;}EXPORT_SYMBOL(_cond_resched);#endif

__mutex_trylock_fast()函数调用atomic_long_cmpxchg_acquire()函数判断lock->owner的值是否等于0，如果等于0，则直接将当前线程的task struct的指针赋值给lock->owner，表示该mutex锁已经被当前线程持有。如果lock->owner的值不等于0，则表示该mutex锁已经被其他线程持有或者锁正在传递给top waiter线程，当前线程需要阻塞等待。上面描述的操作（比较和赋值）都是原子操作，不会有任何指令插入。

static __always_inline bool __mutex_trylock_fast(struct mutex *lock){ unsigned long curr = (unsigned long)current; if (!atomic_long_cmpxchg_acquire(&lock- >owner, 0UL, curr))  return true; return false;}

慢速获取mutex锁的路径就是__mutex_lock_common()函数，所谓慢速其实就是阻塞当前线程，将current task挂入mutex的等待队列的尾部。让所有等待mutex的任务按照时间的先后顺序排列起来，当mutex被释放的时候，会首先唤醒队首的任务，即最先等待的任务最先被唤醒。此外，在向空队列插入第一个任务的时候，会给mutex flag设置上MUTEX_FLAG_WAITERS标记，表示已经有任务在等待这个mutex锁了。

static noinline void __sched__mutex_lock_slowpath(struct mutex *lock){ __mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);}static int __sched__mutex_lock(struct mutex *lock, long state, unsigned int subclass,      struct lockdep_map *nest_lock, unsigned long ip){ return __mutex_lock_common(lock, state, subclass, nest_lock, ip, NULL, false);}static __always_inline int __sched__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,      struct lockdep_map *nest_lock, unsigned long ip,      struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx){ struct mutex_waiter waiter; bool first = false; struct ww_mutex *ww; int ret; might_sleep(); ww = container_of(lock, struct ww_mutex, base); if (use_ww_ctx && ww_ctx) {  if (unlikely(ww_ctx == READ_ONCE(ww- >ctx)))   return -EALREADY; } preempt_disable(); mutex_acquire_nest(&lock- >dep_map, subclass, 0, nest_lock, ip); if (__mutex_trylock(lock) ||     mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, NULL)) {  /* got the lock, yay! */  lock_acquired(&lock- >dep_map, ip);  if (use_ww_ctx && ww_ctx)   ww_mutex_set_context_fastpath(ww, ww_ctx);  preempt_enable();  return 0; } spin_lock(&lock- >wait_lock); /*  * After waiting to acquire the wait_lock, try again.  */ if (__mutex_trylock(lock)) {  if (use_ww_ctx && ww_ctx)   __ww_mutex_wakeup_for_backoff(lock, ww_ctx);  goto skip_wait; } debug_mutex_lock_common(lock, &waiter); debug_mutex_add_waiter(lock, &waiter, current); lock_contended(&lock- >dep_map, ip); if (!use_ww_ctx) {  /* add waiting tasks to the end of the waitqueue (FIFO): */  list_add_tail(&waiter.list, &lock- >wait_list);#ifdef CONFIG_DEBUG_MUTEXES  waiter.ww_ctx = MUTEX_POISON_WW_CTX;#endif } else {  /* Add in stamp order, waking up waiters that must back off. */  ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);  if (ret)   goto err_early_backoff;  waiter.ww_ctx = ww_ctx; } waiter.task = current; if (__mutex_waiter_is_first(lock, &waiter))  __mutex_set_flag(lock, MUTEX_FLAG_WAITERS); set_current_state(state); for (;;) {  /*   * Once we hold wait_lock, we"re serialized against   * mutex_unlock() handing the lock off to us, do a trylock   * before testing the error conditions to make sure we pick up   * the handoff.   */  if (__mutex_trylock(lock))   goto acquired;  /*   * Check for signals and wound conditions while holding   * wait_lock. This ensures the lock cancellation is ordered   * against mutex_unlock() and wake-ups do not go missing.   */  if (unlikely(signal_pending_state(state, current))) {   ret = -EINTR;   goto err;  }  if (use_ww_ctx && ww_ctx && ww_ctx- >acquired > 0) {   ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);   if (ret)    goto err;  }  spin_unlock(&lock- >wait_lock);  schedule_preempt_disabled();  /*   * ww_mutex needs to always recheck its position since its waiter   * list is not FIFO ordered.   */  if ((use_ww_ctx && ww_ctx) || !first) {   first = __mutex_waiter_is_first(lock, &waiter);   if (first)    __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);  }  set_current_state(state);  /*   * Here we order against unlock; we must either see it change   * state back to RUNNING and fall through the next schedule(),   * or we must see its unlock and acquire.   */  if (__mutex_trylock(lock) ||      (first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, &waiter)))   break;  spin_lock(&lock- >wait_lock); } spin_lock(&lock- >wait_lock);acquired: __set_current_state(TASK_RUNNING); mutex_remove_waiter(lock, &waiter, current); if (likely(list_empty(&lock- >wait_list)))  __mutex_clear_flag(lock, MUTEX_FLAGS); debug_mutex_free_waiter(&waiter);skip_wait: /* got the lock - cleanup and rejoice! */ lock_acquired(&lock- >dep_map, ip); if (use_ww_ctx && ww_ctx)  ww_mutex_set_context_slowpath(ww, ww_ctx); spin_unlock(&lock- >wait_lock); preempt_enable(); return 0;err: __set_current_state(TASK_RUNNING); mutex_remove_waiter(lock, &waiter, current);err_early_backoff: spin_unlock(&lock- >wait_lock); debug_mutex_free_waiter(&waiter); mutex_release(&lock- >dep_map, 1, ip); preempt_enable(); return ret;}

释放锁流程分析

释放锁的流程也分快速释放和慢速释放两种路径

void __sched mutex_unlock(struct mutex *lock){#ifndef CONFIG_DEBUG_LOCK_ALLOC if (__mutex_unlock_fast(lock))  return;#endif __mutex_unlock_slowpath(lock, _RET_IP_);}

如果一个线程获取了mutex锁之后，没有其他的线程试图获取，此时的mutex的owner成员就是该线程的task struct地址，并且所有的mutex flag都没有被设置。这时候只需要将mutex的owner成员清零即可，不需要其它操作，这就是快速释放锁的路径。

static __always_inline bool __mutex_unlock_fast(struct mutex *lock){ unsigned long curr = (unsigned long)current; if (atomic_long_cmpxchg_release(&lock- >owner, curr, 0UL) == curr)  return true; return false;}

如果有其他线程在竞争该mutex锁，这时候就会进入慢速释放锁路径，慢速释放锁路径的逻辑分成两段：一段是释放mutex锁，另外一段是唤醒top waiter线程。

static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip){ struct task_struct *next = NULL; DEFINE_WAKE_Q(wake_q); unsigned long owner; mutex_release(&lock- >dep_map, 1, ip); /*  * Release the lock before (potentially) taking the spinlock such that  * other contenders can get on with things ASAP.  *  * Except when HANDOFF, in that case we must not clear the owner field,  * but instead set it to the top waiter.  */ owner = atomic_long_read(&lock- >owner); for (;;) {  unsigned long old;#ifdef CONFIG_DEBUG_MUTEXES  DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);  DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);#endif  if (owner & MUTEX_FLAG_HANDOFF)   break;  old = atomic_long_cmpxchg_release(&lock- >owner, owner,        __owner_flags(owner));  if (old == owner) {   if (owner & MUTEX_FLAG_WAITERS)    break;   return;  }  owner = old; } spin_lock(&lock- >wait_lock); debug_mutex_unlock(lock); if (!list_empty(&lock- >wait_list)) {  /* get the first entry from the wait-list: */  struct mutex_waiter *waiter =   list_first_entry(&lock- >wait_list,      struct mutex_waiter, list);  next = waiter- >task;  debug_mutex_wake_waiter(lock, waiter);  wake_q_add(&wake_q, next); } if (owner & MUTEX_FLAG_HANDOFF)  __mutex_handoff(lock, next); spin_unlock(&lock- >wait_lock); wake_up_q(&wake_q);}

总结

本篇主要介绍了mutex互斥锁的使用注意事项，介绍了mutex的主要函数接口以及获取锁的流程分析和释放锁的流程分析。通过本文的学习，我们基本可以了解了mutex的实现机制和使用方法。

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