Linux 内核编程基本功之内核同步与互斥锁mutex
背景
在使用PostgreSQL实现同步流复制时，在主节点发现有大量的mutex，导致了写并发被限制。
本文为转载文章
http://blog.csdn.net/cug_fish_2009/article/details/6126414
Pro-II、内核同步与互斥锁
1、理解互斥锁？
互斥锁的使用也是保持内核临界区的同步的，互斥锁可以说源于信号量，信号量设置计数器可以容许n个进程并发的访问临界区，而互斥锁不行，只能容许每次一个进程访问，也就是计数器值为1的信号量，
可以这么理解。互斥锁和自旋锁有不同（显然的），互斥锁在中断处理程序中和可延迟函数中都不能使用，因为它是可以睡眠的，只能在进程上下文或者软中断上下文才能使用。
2、如何实现互斥锁？
2-1、结构体：
2-1-1、mutex结构体：
struct mutex {
/* 1: unlocked, 0: locked, negative: locked, possible waiters */
atomic_t count;
spinlock_t wait_lock;
struct list_head wait_list;
//我们关心上面三个变量
#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
};
2-1-2、mutex_waiter结构体：
struct mutex_waiter {
struct list_head list;
struct task_struct *task;
#ifdef CONFIG_DEBUG_MUTEXES
void *magic;
#endif
};
2-2、方法：
2-2-1、初始化：
# define mutex_init(mutex) /
do { /
static struct lock_class_key __key; /
/
__mutex_init((mutex), #mutex, &__key); /
} while (0)
void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
atomic_set(&lock->count, 1); //设置计数器
spin_lock_init(&lock->wait_lock); //初始化自旋锁
INIT_LIST_HEAD(&lock->wait_list); //初始化等待队列
mutex_clear_owner(lock);
debug_mutex_init(lock, name, key);
}
2-2-2、锁住：
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);
}
#define __mutex_fastpath_lock(count, fail_fn) /
do { /
unsigned int dummy; /
/
typecheck(atomic_t *, count); /
typecheck_fn(void (*)(atomic_t *), fail_fn); /
/
asm volatile(LOCK_PREFIX " decl (%%eax)/n" /
" jns 1f /n" /
//如果count>=0，即之前的count>0（其实也只有一种情况的count==1）
//说明资源空闲着，可以直接使用
" call " #fail_fn "/n" /
//如果之前的count<=0，说明资源忙或者已经有进程在等待
//于是我们要睡眠，也就是将本进程加入该互斥锁等待队列中
//调用的是fail_fn这个函数
"1:/n" /
: "=a" (dummy) /
: "a" (count) /
: "memory", "ecx", "edx"); /
} while (0)
static __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count)
{
struct mutex *lock = container_of(lock_count, struct mutex, count);
__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_);
}
static inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
unsigned long ip)
{
struct task_struct *task = current;
struct mutex_waiter waiter;
unsigned long flags;
preempt_disable(); //禁止内核抢占
mutex_acquire(&lock->dep_map, subclass, 0, ip);
//忽略下面的定义
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
/*
* Optimistic spinning.
*
* We try to spin for acquisition when we find that there are no
* pending waiters and the lock owner is currently running on a
* (different) CPU.
*
* The rationale is that if the lock owner is running, it is likely to
* release the lock soon.
*
* Since this needs the lock owner, and this mutex implementation
* doesn't track the owner atomically in the lock field, we need to
* track it non-atomically.
*
* We can't do this for DEBUG_MUTEXES because that relies on wait_lock
* to serialize everything.
*/
for (;;) {
struct thread_info *owner;
/*
* If we own the BKL, then don't spin. The owner of
* the mutex might be waiting on us to release the BKL.
*/
if (unlikely(current->lock_depth >= 0))
break;
/*
* If there's an owner, wait for it to either
* release the lock or go to sleep.
*/
owner = ACCESS_ONCE(lock->owner);
if (owner && !mutex_spin_on_owner(lock, owner))
break;
if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
lock_acquired(&lock->dep_map, ip);
mutex_set_owner(lock);
preempt_enable();
return 0;
}
/*
* When there's no owner, we might have preempted between the
* owner acquiring the lock and setting the owner field. If
* we're an RT task that will live-lock because we won't let
* the owner complete.
*/
if (!owner && (need_resched() || rt_task(task)))
break;
/*
* The cpu_relax() call is a compiler barrier which forces
* everything in this loop to be re-loaded. We don't need
* memory barriers as we'll eventually observe the right
* values at the cost of a few extra spins.
*/
cpu_relax();
}
#endif
spin_lock_mutex(&lock->wait_lock, flags); //自旋锁一下
debug_mutex_lock_common(lock, &waiter);
debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
/* add waiting tasks to the end of the waitqueue (FIFO): */
list_add_tail(&waiter.list, &lock->wait_list);
//将本进程放入互斥锁FIFO队列中
waiter.task = task;
if (atomic_xchg(&lock->count, -1) == 1)
goto done;
//有可能在调用这个函数的途中互斥锁释放了，在进入循环前检查一遍
//如果果真释放了，就直接进入临界区
lock_contended(&lock->dep_map, ip);
for (;;) {
/*
* Lets try to take the lock again - this is needed even if
* we get here for the first time (shortly after failing to
* acquire the lock), to make sure that we get a wakeup once
* it's unlocked. Later on, if we sleep, this is the
* operation that gives us the lock. We xchg it to -1, so
* that when we release the lock, we properly wake up the
* other waiters:
*/
if (atomic_xchg(&lock->count, -1) == 1)
break;
//如果发现互斥锁被释放了，则终止循环
/*
* got a signal? (This code gets eliminated in the
* TASK_UNINTERRUPTIBLE case.)
*/
if (unlikely(signal_pending_state(state, task))) {
mutex_remove_waiter(lock, &waiter,
task_thread_info(task));
mutex_release(&lock->dep_map, 1, ip);
spin_unlock_mutex(&lock->wait_lock, flags);
debug_mutex_free_waiter(&waiter);
preempt_enable();
return -EINTR;
}
//执行的可能行极小
__set_task_state(task, state);
//设置本进程状态为不可中断（TASK_UNINTERRUPTIBLE）
/* didnt get the lock, go to sleep: */
spin_unlock_mutex(&lock->wait_lock, flags);
//释放自旋锁
preempt_enable_no_resched();
schedule();
//现在切换进程，进入睡眠阶段
//一旦被唤醒
preempt_disable();
spin_lock_mutex(&lock->wait_lock, flags);
}
done:
lock_acquired(&lock->dep_map, ip);
/* got the lock - rejoice! */
mutex_remove_waiter(lock, &waiter, current_thread_info());
//将本进程从互斥锁FIFO队列中移除
mutex_set_owner(lock);
/* set it to 0 if there are no waiters left: */
if (likely(list_empty(&lock->wait_list)))
atomic_set(&lock->count, 0);
//注意，如果互斥锁等待队列中没有等待的进程，则将count置0
//表示资源正忙，但是没有进程等待
//这里解释一下count的取值
//1：表示资源空闲，0：表示资源忙，但没有等待资源的进程，-1：表示资源忙，且有等待资源的进程
spin_unlock_mutex(&lock->wait_lock, flags);
debug_mutex_free_waiter(&waiter);
preempt_enable();
//恢复一切，正常执行代码
return 0;
}
2-2-3、释放锁：
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);
}
#define __mutex_fastpath_unlock(count, fail_fn) /
do { /
unsigned int dummy; /
/
typecheck(atomic_t *, count); /
typecheck_fn(void (*)(atomic_t *), fail_fn); /
/
asm volatile(LOCK_PREFIX " incl (%%eax)/n" /
" jg 1f/n" /
//如果发现count>0，即之前的count>=0，说明没有等待该资源的进程
//那么无需去唤醒另外的进程
" call " #fail_fn "/n" /
//否则唤醒另外的进程
"1:/n" /
: "=a" (dummy) /
: "a" (count) /
: "memory", "ecx", "edx"); /
} while (0)
static __used noinline void
__mutex_unlock_slowpath(atomic_t *lock_count)
{
__mutex_unlock_common_slowpath(lock_count, 1);
}
static inline void
__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
{
struct mutex *lock = container_of(lock_count, struct mutex, count);
unsigned long flags;
spin_lock_mutex(&lock->wait_lock, flags);
mutex_release(&lock->dep_map, nested, _RET_IP_);
debug_mutex_unlock(lock);
/*
* some architectures leave the lock unlocked in the fastpath failure
* case, others need to leave it locked. In the later case we have to
* unlock it here
*/
if (__mutex_slowpath_needs_to_unlock())
atomic_set(&lock->count, 1);
//这里在唤醒的时候将count置1，表示资源空闲
//这里看似与count值的说法有错，因为现在可能还有别的进程等待该资源
//但是当唤醒到另外的进程中会看到，结合mutex_lock中 atomic_xchg函数
//而解锁函数如何知道还有没有等待的进程，判断等待队列是否为空就搞定了
if (!list_empty(&lock->wait_list)) {
/* get the first entry from the wait-list: */
struct mutex_waiter *waiter =
list_entry(lock->wait_list.next,
struct mutex_waiter, list);
//找到队列中排在前面的那个进程
debug_mutex_wake_waiter(lock, waiter);
wake_up_process(waiter->task); //唤醒那个进程
}
spin_unlock_mutex(&lock->wait_lock, flags);
}
#define __mutex_slowpath_needs_to_unlock() 1
互斥锁是比较难理解的，希望多花点时间，多看点资料，结合一些具体情况。