PID

/*
 * What is struct pid?
 *
 * A struct pid is the kernel's internal notion of a process identifier.
 * It refers to individual tasks, process groups, and sessions.  While
 * there are processes attached to it the struct pid lives in a hash
 * table, so it and then the processes that it refers to can be found
 * quickly from the numeric pid value.  The attached processes may be
 * quickly accessed by following pointers from struct pid.

 *
 * pid结构是进程id的内核内部概念。它代表单独任务，进程组和会话。因为进程与它紧密相连且pid结构存储在一个哈希表中，

 * 所以它代表的进程可以很容易由pid值快速查找到。同时由pid结构的指针可以很快的访问连接的进程。

 *
 * Storing pid_t values in the kernel and refering to them later has a
 * problem.  The process originally with that pid may have exited and the
 * pid allocator wrapped, and another process could have come along
 * and been assigned that pid.
 *

 * 在内核中储存pid_t值之后访问它们产生一个问题。原先与该pid相关的进程已经推出，且pid分配器已经清除了该pid，

 * 其他进程可能紧接着分配了该pid。

 *
 * Referring to user space processes by holding a reference to struct
 * task_struct has a problem.  When the user space process exits
 * the now useless task_struct is still kept.  A task_struct plus a
 * stack consumes around 10K of low kernel memory.  More precisely
 * this is THREAD_SIZE + sizeof(struct task_struct).  By comparison
 * a struct pid is about 64 bytes.

 *
 * 谈到用户空间进程持有一个task_struct结构的引用会产生一个问题。当用户空间进程退出，无用的task_struct仍然存在。

 * task_struct增加了大约10k的低内核存储的栈消耗。更精确的是THREAD_SIZE + sizeof(struct task_struct)大小。

 *
 * Holding a reference to struct pid solves both of these problems.
 * It is small so holding a reference does not consume a lot of
 * resources, and since a new struct pid is allocated when the numeric pid
 * value is reused (when pids wrap around) we don't mistakenly refer to new
 * processes.
 */


/*
 * struct upid is used to get the id of the struct pid, as it is
 * seen in particular namespace. Later the struct pid is found with
 * find_pid_ns() using the int nr and struct pid_namespace *ns.
 */

struct upid {
    /* Try to keep pid_chain in the same cacheline as nr for find_vpid */
    int nr;
    struct pid_namespace *ns;
    struct hlist_node pid_chain;
};

struct pid
{
    atomic_t count;
    unsigned int level;
    /* lists of tasks that use this pid */
    struct hlist_head tasks[PIDTYPE_MAX];
    struct rcu_head rcu;
    struct upid numbers[1];
};

 

enum pid_type
{
 PIDTYPE_PID,
 PIDTYPE_PGID,
 PIDTYPE_SID,
 PIDTYPE_MAX
};

 

struct pid_link
{
 struct hlist_node node;
 struct pid *pid;
};

 

struct task_struct

{

...................................

 pid_t pid;
 pid_t tgid;

/* PID/PID hash table linkage. */
 struct pid_link pids[PIDTYPE_MAX];

..................................

};

 

该图片引自《Professional linux kernel architecture》图中的mode应为node