进程管理API之find_get_pid

struct pid *find_get_pid(pid_t nr)根据进程号pid_t nr nr得到进程描述符struct pid ，并将进程描述符中的字段count加1这里的进程描述符定义如下：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];};其使用的例子如下：static int proc_do_cad_pid(struct ctl_table *table, int write,   void __user *buffer, size_t *lenp, loff_t *ppos){struct pid *new_pid;pid_t tmp;int r;tmp = pid_vnr(cad_pid);r = __do_proc_dointvec(&tmp, table, write, buffer,       lenp, ppos, NULL, NULL);if (r || !write)return r;new_pid = find_get_pid(tmp);if (!new_pid)return -ESRCH;put_pid(xchg(&cad_pid, new_pid));return 0;}其源码分析如下：struct pid *find_get_pid(pid_t nr){struct pid *pid;rcu_read_lock();pid = get_pid(find_vpid(nr));rcu_read_unlock();return pid;}这里通过find_vpid(nr) 来找到进程描述符，然后通过get_pid来让count字段加1首先看get_pid来让count字段加1static inline struct pid *get_pid(struct pid *pid){if (pid)atomic_inc(&pid->count);return pid;}如果pid 不为NULL的话，就通过atomic_inc 来让count字段加1其次看看通过find_vpid 找到进程描述符struct pid *find_vpid(int nr){//通过task_active_pid_ns 找到当前进程的namespace，这里有个小疑问，为啥nr一定会在当前的namesapce内呢?return find_pid_ns(nr, task_active_pid_ns(current));}struct pid *find_pid_ns(int nr, struct pid_namespace *ns){struct upid *pnr;//kernel中所有的pid都是放在pid_hash 中的，然后根据nr和ns找到对应的pid_hash的list，最后遍历这个pid_hash，找到进程描述符要符合两个条件，一个是进程描述符的pid 要等于nr，另外一个是进程描述符的namaspce 要得到ns，也就是当前进程的namespacehlist_for_each_entry_rcu(pnr,&pid_hash[pid_hashfn(nr, ns)], pid_chain)if (pnr->nr == nr && pnr->ns == ns)//这里根据struct pid 中成员变量level 返回struct pid的指针return container_of(pnr, struct pid,numbers[ns->level]);return NULL;}