Linux 之mutex 源码分析

mutex相关的函数并不是linux kernel实现的，而是glibc实现的，源码位于nptl目录下。

http://ftp.gnu.org/pub/gnu/glibc/glibc-2.3.5.tar.gz

首先说数据结构：

typedef union

{

struct

{

int __lock;

unsigned int __count;

int __owner;

unsigned int __nusers;

/* KIND must stay at this position in the structure to maintain

binary compatibility. */

int __kind;

int __spins;

} __data;

char __size[__SIZEOF_PTHREAD_MUTEX_T];

long int __align;

} pthread_mutex_t;

int __lock; 资源竞争引用计数

int __kind; 锁类型，init 函数中mutexattr 参数传递，该参数可以为NULL，一般为 PTHREAD_MUTEX_NORMAL

结构体其他元素暂时不了解，以后更新。

/*nptl/pthread_mutex_init.c*/

int

__pthread_mutex_init (mutex, mutexattr)

pthread_mutex_t *mutex;

const pthread_mutexattr_t *mutexattr;

{

const struct pthread_mutexattr *imutexattr;

assert (sizeof (pthread_mutex_t) <= __SIZEOF_PTHREAD_MUTEX_T);

imutexattr = (const struct pthread_mutexattr *) mutexattr ?: &default_attr;

/* Clear the whole variable. */

memset (mutex, '\0', __SIZEOF_PTHREAD_MUTEX_T);

/* Copy the values from the attribute. */

mutex->__data.__kind = imutexattr->mutexkind & ~0x80000000;

/* Default values: mutex not used yet. */

// mutex->__count = 0; already done by memset

// mutex->__owner = 0; already done by memset

// mutex->__nusers = 0; already done by memset

// mutex->__spins = 0; already done by memset

return 0;

}

init函数就比较简单了，将mutex结构体清零，设置结构体中__kind属性。

/*nptl/pthread_mutex_lock.c*/

int

__pthread_mutex_lock (mutex)

pthread_mutex_t *mutex;

{

assert (sizeof (mutex->__size) >= sizeof (mutex->__data));

pid_t id = THREAD_GETMEM (THREAD_SELF, tid);

switch (__builtin_expect (mutex->__data.__kind, PTHREAD_MUTEX_TIMED_NP))

{

…

default:

/* Correct code cannot set any other type. */

case PTHREAD_MUTEX_TIMED_NP:

simple:

/* Normal mutex. */

LLL_MUTEX_LOCK (mutex->__data.__lock);

break;

…

}

/* Record the ownership. */

assert (mutex->__data.__owner == 0);

mutex->__data.__owner = id;

#ifndef NO_INCR

++mutex->__data.__nusers;

#endif

return 0;

}

该函数主要是调用LLL_MUTEX_LOCK，省略部分为根据mutex结构体__kind属性不同值做些处理。

宏定义函数LLL_MUTEX_LOCK最终调用，将结构体mutex的__lock属性作为参数传递进来

#define __lll_mutex_lock(futex) \

((void) ({ \

int *__futex = (futex); \

if (atomic_compare_and_exchange_bool_acq (__futex, 1, 0) != 0) \

__lll_lock_wait (__futex); \

}))

atomic_compare_and_exchange_bool_acq (__futex, 1, 0)宏定义为：

#define atomic_compare_and_exchange_bool_acq(mem, newval, oldval) \

({ __typeof (mem) __gmemp = (mem); \

__typeof (*mem) __gnewval = (newval); \

*__gmemp == (oldval) ? (*__gmemp = __gnewval, 0) : 1; })

这个宏实现的功能是：

如果mem的值等于oldval，则把newval赋值给mem，放回0，否则不做任何处理，返回1.

由此可以看出，当mutex锁限制的资源没有竞争时，__lock 属性被置为1，并返回0，不会调用__lll_lock_wait (__futex); 当存在竞争时，再次调用lock函数，该宏不做任何处理，返回1，调用__lll_lock_wait (__futex);

void

__lll_lock_wait (int *futex)

{

int oldval = atomic_compare_and_exchange_val_acq (futex, 2, 1);

if (oldval != 0)

lll_futex_wait (futex, 2);

}

while (atomic_compare_and_exchange_bool_acq (futex, 2, 0) != 0);

}

atomic_compare_and_exchange_val_acq (futex, 2, 1); 宏定义：

/* The only basic operation needed is compare and exchange. */

#define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \

({ __typeof (mem) __gmemp = (mem); \

__typeof (*mem) __gret = *__gmemp; \

__typeof (*mem) __gnewval = (newval); \

if (__gret == (oldval)) \

*__gmemp = __gnewval; \

__gret; })

这个宏实现的功能是，当mem等于oldval时，将mem置为newval，始终返回mem原始值。

此时，futex等于1，futex将被置为2，并且返回1. 进而调用

lll_futex_wait (futex, 2);

#define lll_futex_timed_wait(ftx, val, timespec) \

({ \

DO_INLINE_SYSCALL(futex, 4, (long) (ftx), FUTEX_WAIT, (int) (val), \

(long) (timespec)); \

_r10 == -1 ? -_retval : _retval; \

})

该宏对于不同的平台架构会用不同的实现，采用汇编语言实现系统调用。不过确定的是调用了Linux kernel的futex系统调用。

futex在linux kernel的实现位于：kernel/futex.c

SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,

struct timespec __user *, utime, u32 __user *, uaddr2,

u32, val3)

{

struct timespec ts;

ktime_t t, *tp = NULL;

u32 val2 = 0;

int cmd = op & FUTEX_CMD_MASK;

if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||

cmd == FUTEX_WAIT_BITSET ||

cmd == FUTEX_WAIT_REQUEUE_PI)) {

if (copy_from_user(&ts, utime, sizeof(ts)) != 0)

return -EFAULT;

if (!timespec_valid(&ts))

return -EINVAL;

t = timespec_to_ktime(ts);

if (cmd == FUTEX_WAIT)

t = ktime_add_safe(ktime_get(), t);

tp = &t;

}

* requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.

* number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.

if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||

cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)

val2 = (u32) (unsigned long) utime;

return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);

}

futex具有六个形参，pthread_mutex_lock最终只关注了前四个。futex函数对参数进行判断和转化之后，直接调用do_futex。

long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,

u32 __user *uaddr2, u32 val2, u32 val3)

{

int clockrt, ret = -ENOSYS;

int cmd = op & FUTEX_CMD_MASK;

int fshared = 0;

if (!(op & FUTEX_PRIVATE_FLAG))

fshared = 1;

clockrt = op & FUTEX_CLOCK_REALTIME;

if (clockrt && cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)

return -ENOSYS;

switch (cmd) {

case FUTEX_WAIT:

val3 = FUTEX_BITSET_MATCH_ANY;

case FUTEX_WAIT_BITSET:

ret = futex_wait(uaddr, fshared, val, timeout, val3, clockrt);

break;

…

default:

ret = -ENOSYS;

}

return ret;

}

省略部分为对其他cmd的处理，pthread_mutex_lock函数最终传入的cmd参数为FUTEX_WAIT，所以在此只关注此分之，分析futex_wait函数的实现。

static int futex_wait(u32 __user *uaddr, int fshared,

u32 val, ktime_t *abs_time, u32 bitset, int clockrt)

{

struct hrtimer_sleeper timeout, *to = NULL;

struct restart_block *restart;

struct futex_hash_bucket *hb;

struct futex_q q;

int ret;

… … //delete parameters check and convertion

retry:

/* Prepare to wait on uaddr. */

ret = futex_wait_setup(uaddr, val, fshared, &q, &hb);

if (ret)

goto out;

/* queue_me and wait for wakeup, timeout, or a signal. */

futex_wait_queue_me(hb, &q, to);

… … //other handlers

return ret;

}

futex_wait_setup 将线程放进休眠队列中，

futex_wait_queue_me(hb, &q, to);将本线程休眠，等待唤醒。

唤醒后，__lll_lock_wait函数中的while (atomic_compare_and_exchange_bool_acq (futex, 2, 0) != 0); 语句将被执行，由于此时futex在pthread_mutex_unlock中置为0，所以atomic_compare_and_exchange_bool_acq (futex, 2, 0)语句将futex置为2，返回0. 退出循环，访问用户控件的临界资源。

/*nptl/pthread_mutex_unlock.c*/

int

internal_function attribute_hidden

__pthread_mutex_unlock_usercnt (mutex, decr)

pthread_mutex_t *mutex;

int decr;

{

switch (__builtin_expect (mutex->__data.__kind, PTHREAD_MUTEX_TIMED_NP))

{

… …

default:

/* Correct code cannot set any other type. */

case PTHREAD_MUTEX_TIMED_NP:

case PTHREAD_MUTEX_ADAPTIVE_NP:

/* Normal mutex. Nothing special to do. */

break;

}

/* Always reset the owner field. */

mutex->__data.__owner = 0;

if (decr)

/* One less user. */

--mutex->__data.__nusers;

/* Unlock. */

lll_mutex_unlock (mutex->__data.__lock);

return 0;

}

省略部分是针对不同的__kind属性值做的一些处理，最终调用 lll_mutex_unlock。

该宏函数最终的定义为：

#define __lll_mutex_unlock(futex) \

((void) ({ \

int *__futex = (futex); \

int __val = atomic_exchange_rel (__futex, 0); \

if (__builtin_expect (__val > 1, 0)) \

lll_futex_wake (__futex, 1); \

}))

atomic_exchange_rel (__futex, 0);宏为：

#define atomic_exchange_rel(mem, value) \

(__sync_synchronize (), __sync_lock_test_and_set (mem, value))

实现功能为：将mem设置为value，返回原始mem值。

__builtin_expect (__val > 1, 0) 是编译器优化语句，告诉编译器期望值，也就是大多数情况下__val > 1 ？是0，其逻辑判断依然为if(__val > 1)为真的话执行 lll_futex_wake。

现在分析，在资源没有被竞争的情况下，__futex 为1，那么返回值__val则为1，那么 lll_futex_wake (__futex, 1); 不会被执行，不产生系统调用。当资源产生竞争的情况时，根据对pthread_mutex_lock 函数的分析，__futex为2， __val则为2，执行 lll_futex_wake (__futex, 1); 从而唤醒等在临界资源的线程。

lll_futex_wake (__futex, 1); 最终会调动同一个系统调用，即futex, 只是传递的cmd参数为FUTEX_WAKE。

在linux kernel的futex实现中，调用

static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)

{

struct futex_hash_bucket *hb;

struct futex_q *this, *next;

struct plist_head *head;

union futex_key key = FUTEX_KEY_INIT;

int ret;

if (!bitset)

return -EINVAL;

ret = get_futex_key(uaddr, fshared, &key);

if (unlikely(ret != 0))

goto out;

hb = hash_futex(&key);

spin_lock(&hb->lock);

head = &hb->chain;

plist_for_each_entry_safe(this, next, head, list) {

if (match_futex (&this->key, &key)) {

if (this->pi_state || this->rt_waiter) {

ret = -EINVAL;

break;

}

/* Check if one of the bits is set in both bitsets */

if (!(this->bitset & bitset))

continue;

wake_futex(this);

if (++ret >= nr_wake)

break;

}

spin_unlock(&hb->lock);

put_futex_key(fshared, &key);

out:

return ret;

}

该函数遍历在该mutex上休眠的所有线程，调用wake_futex进行唤醒，

static void wake_futex(struct futex_q *q)

{

struct task_struct *p = q->task;

* We set q->lock_ptr = NULL _before_ we wake up the task. If

* a non futex wake up happens on another CPU then the task

* might exit and p would dereference a non existing task

* struct. Prevent this by holding a reference on p across the

* wake up.

get_task_struct(p);

plist_del(&q->list, &q->list.plist);

* The waiting task can free the futex_q as soon as

* q->lock_ptr = NULL is written, without taking any locks. A

* memory barrier is required here to prevent the following

* store to lock_ptr from getting ahead of the plist_del.

smp_wmb();

q->lock_ptr = NULL;

wake_up_state(p, TASK_NORMAL);

put_task_struct(p);

}

wake_up_state(p, TASK_NORMAL); 的实现位于kernel/sched.c中，属于linux进程调度的技术。