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task_struct 是内核用来表示进程的，包含进程的所有信息，该结构体定义在incluce\linux\sched.h里

首先介绍一下状态信息

volatile long state

这个字段存储的是进程当前的状态 

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volatile关键字是一种类型修饰符，用它声明的类型变量表示可以被某些编译器未知的因素更改，比如：操作系统、硬件或者其它线程等。遇到这个关键字声明的变量，编译器对访问该变量的代码就不再进行优化，从而可以提供对特殊地址的稳定访问。

具体参见：

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/*

 * task state bitmask. note! these bits are also

 * encoded in fs/proc/array.c: get_task_state().

 *

 * we have two separate sets of flags: task->state

 * is about runnability, while task->exit_state are

 * about the task exiting. confusing, but this way

 * modifying one set can't modify the other one by

 * mistake.

 */

下面四种表示进程的状态

#define task_running                0

#define task_interruptible        1

#define task_uninterruptible        2

#define __task_stopped        4

#define __task_traced                8

/* in tsk->exit_state */

#define exit_zombie                16

#define exit_dead                32

/* in tsk->state again */

#define task_dead                64

#define task_wakekill                128

#define task_waking                256

#define task_state_max                512

操作状态的宏

/* convenience macros for the sake of set_task_state */

#define task_killable                (task_wakekill | task_uninterruptible)

#define task_stopped                (task_wakekill | __task_stopped)

#define task_traced                (task_wakekill | __task_traced)

/* convenience macros for the sake of wake_up */

#define task_normal                (task_interruptible | task_uninterruptible)

#define task_all                (task_normal | __task_stopped | __task_traced)

/* get_task_state() */

#define task_report                (task_running | task_interruptible | \

 task_uninterruptible | __task_stopped | \

 __task_traced)

#define task_is_traced(task)        ((task->state & __task_traced) != 0)

#define task_is_stopped(task)        ((task->state & __task_stopped) != 0)

#define task_is_dead(task)        ((task)->exit_state != 0)

#define task_is_stopped_or_traced(task)        \

((task->state & (__task_stopped | __task_traced)) != 0)

#define task_contributes_to_load(task)        \

((task->state & task_uninterruptible) != 0 && \

 (task->flags & pf_freezing) == 0)

#define __set_task_state(tsk, state_value)                \

do { (tsk)->state = (state_value); } while (0)

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这里do{...}while 的作用是防止在把宏用在if...else语句中时，会导致编译不通过。

具体跟过应用参见

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#define set_task_state(tsk, state_value) set_mb((tsk)->state, (state_value))

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set_mb宏：

#define set_mb(var, value)  do { var = value; mb(); } while (0)

mb()宏：

#define mb() asm volatile("mfence":::"memory")

mb()宏在不同的系统架构中时不同的，这里取的是x86的架构下的代码

mfence是内存边界的意思，这条指令的作用是在mfence指令前的读写操作当必须在mfence指令后的读写操作前完成。 

在这里的作用是，保证var=value的操作在其后续指令之前完成，防止value值被后面的代码改写

具体内存边界相关内容参见

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设置进程当前状态的宏

/*

 * set_current_state() includes a barrier so that the write of current->state

 * is correctly serialised wrt the caller's subsequent test of whether to

 * actually sleep:

 *

 *set_current_state(task_uninterruptible);

 *if (do_i_need_to_sleep())

 *schedule();

 *

 * if the caller does not need such serialisation then use __set_current_state()

 */

#define __set_current_state(state_value)\

do { current->state = (state_value); } while (0)

#define set_current_state(state_value)\

set_mb(current->state, (state_value))

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这里定义了两个宏:__set_current_state(state_value)和set_current_state(state_value)

前者不带内存屏障，后者是带内存屏障的，即是前者的指令是串行执行的，后者可能是乱序的

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