include/linux/pid.h

   1 #ifndef _LINUX_PID_H
   2 #define _LINUX_PID_H
   3
   4 #include <linux/rcupdate.h>
   5
   6 enum pid_type
   7 {
   8         PIDTYPE_PID,
   9         PIDTYPE_PGID,
  10         PIDTYPE_SID,
  11         PIDTYPE_MAX
  12 };
  13
  14 /*
  15  * What is struct pid?
  16  *
  17  * A struct pid is the kernel's internal notion of a process identifier.
  18  * It refers to individual tasks, process groups, and sessions.  While
  19  * there are processes attached to it the struct pid lives in a hash
  20  * table, so it and then the processes that it refers to can be found
  21  * quickly from the numeric pid value.  The attached processes may be
  22  * quickly accessed by following pointers from struct pid.
  23  *
  24  * Storing pid_t values in the kernel and refering to them later has a
  25  * problem.  The process originally with that pid may have exited and the
  26  * pid allocator wrapped, and another process could have come along
  27  * and been assigned that pid.
  28  *
  29  * Referring to user space processes by holding a reference to struct
  30  * task_struct has a problem.  When the user space process exits
  31  * the now useless task_struct is still kept.  A task_struct plus a
  32  * stack consumes around 10K of low kernel memory.  More precisely
  33  * this is THREAD_SIZE + sizeof(struct task_struct).  By comparison
  34  * a struct pid is about 64 bytes.
  35  *
  36  * Holding a reference to struct pid solves both of these problems.
  37  * It is small so holding a reference does not consume a lot of
  38  * resources, and since a new struct pid is allocated when the numeric
  39  * pid value is reused we don't mistakenly refer to new processes.
  40  */
  41
  42 struct pid
  43 {
  44         atomic_t count;
  45         /* Try to keep pid_chain in the same cacheline as nr for find_pid */
  46         int nr;
  47         struct hlist_node pid_chain;
  48         /* lists of tasks that use this pid */
  49         struct hlist_head tasks[PIDTYPE_MAX];
  50         struct rcu_head rcu;
  51 };
  52
  53 struct pid_link
  54 {
  55         struct hlist_node node;
  56         struct pid *pid;
  57 };
  58
  59 static inline struct pid *get_pid(struct pid *pid)
  60 {
  61         if (pid)
  62                 atomic_inc(&pid->count);
  63         return pid;
  64 }
  65
  66 extern void FASTCALL(put_pid(struct pid *pid));
  67 extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
  68 extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
  69                                                 enum pid_type));
  70
  71 /*
  72  * attach_pid() and detach_pid() must be called with the tasklist_lock
  73  * write-held.
  74  */
  75 extern int FASTCALL(attach_pid(struct task_struct *task,
  76                                 enum pid_type type, int nr));
  77
  78 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
  79
  80 /*
  81  * look up a PID in the hash table. Must be called with the tasklist_lock
  82  * or rcu_read_lock() held.
  83  */
  84 extern struct pid *FASTCALL(find_pid(int nr));
  85
  86 /*
  87  * Lookup a PID in the hash table, and return with it's count elevated.
  88  */
  89 extern struct pid *find_get_pid(int nr);
  90
  91 extern struct pid *alloc_pid(void);
  92 extern void FASTCALL(free_pid(struct pid *pid));
  93
  94 #define pid_next(task, type)                                    \
  95         ((task)->pids[(type)].node.next)
  96
  97 #define pid_next_task(task, type)                               \
  98         hlist_entry(pid_next(task, type), struct task_struct,   \
  99                         pids[(type)].node)
 100
 101
 102 /* We could use hlist_for_each_entry_rcu here but it takes more arguments
 103  * than the do_each_task_pid/while_each_task_pid.  So we roll our own
 104  * to preserve the existing interface.
 105  */
 106 #define do_each_task_pid(who, type, task)                               \
 107         if ((task = find_task_by_pid_type(type, who))) {                \
 108                 prefetch(pid_next(task, type));                         \
 109                 do {
 110
 111 #define while_each_task_pid(who, type, task)                            \
 112                 } while (pid_next(task, type) &&  ({                    \
 113                                 task = pid_next_task(task, type);       \
 114                                 rcu_dereference(task);                  \
 115                                 prefetch(pid_next(task, type));         \
 116                                 1; }) );                                \
 117         }
 118
 119 #endif /* _LINUX_PID_H */