linux/include/linux/pid.h
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   1#ifndef _LINUX_PID_H
   2#define _LINUX_PID_H
   3
   4#include <linux/rcupdate.h>
   5
   6enum 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 referring 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 pid
  39 * value is reused (when pids wrap around) we don't mistakenly refer to new
  40 * processes.
  41 */
  42
  43
  44/*
  45 * struct upid is used to get the id of the struct pid, as it is
  46 * seen in particular namespace. Later the struct pid is found with
  47 * find_pid_ns() using the int nr and struct pid_namespace *ns.
  48 */
  49
  50struct upid {
  51        /* Try to keep pid_chain in the same cacheline as nr for find_vpid */
  52        int nr;
  53        struct pid_namespace *ns;
  54        struct hlist_node pid_chain;
  55};
  56
  57struct pid
  58{
  59        atomic_t count;
  60        unsigned int level;
  61        /* lists of tasks that use this pid */
  62        struct hlist_head tasks[PIDTYPE_MAX];
  63        struct rcu_head rcu;
  64        struct upid numbers[1];
  65};
  66
  67extern struct pid init_struct_pid;
  68
  69struct pid_link
  70{
  71        struct hlist_node node;
  72        struct pid *pid;
  73};
  74
  75static inline struct pid *get_pid(struct pid *pid)
  76{
  77        if (pid)
  78                atomic_inc(&pid->count);
  79        return pid;
  80}
  81
  82extern void put_pid(struct pid *pid);
  83extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  84extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  85
  86extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  87
  88/*
  89 * attach_pid() and detach_pid() must be called with the tasklist_lock
  90 * write-held.
  91 */
  92extern void attach_pid(struct task_struct *task, enum pid_type type,
  93                        struct pid *pid);
  94extern void detach_pid(struct task_struct *task, enum pid_type);
  95extern void change_pid(struct task_struct *task, enum pid_type,
  96                        struct pid *pid);
  97extern void transfer_pid(struct task_struct *old, struct task_struct *new,
  98                         enum pid_type);
  99
 100struct pid_namespace;
 101extern struct pid_namespace init_pid_ns;
 102
 103/*
 104 * look up a PID in the hash table. Must be called with the tasklist_lock
 105 * or rcu_read_lock() held.
 106 *
 107 * find_pid_ns() finds the pid in the namespace specified
 108 * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
 109 *
 110 * see also find_task_by_vpid() set in include/linux/sched.h
 111 */
 112extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 113extern struct pid *find_vpid(int nr);
 114
 115/*
 116 * Lookup a PID in the hash table, and return with it's count elevated.
 117 */
 118extern struct pid *find_get_pid(int nr);
 119extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 120int next_pidmap(struct pid_namespace *pid_ns, unsigned int last);
 121
 122extern struct pid *alloc_pid(struct pid_namespace *ns);
 123extern void free_pid(struct pid *pid);
 124
 125/*
 126 * ns_of_pid() returns the pid namespace in which the specified pid was
 127 * allocated.
 128 *
 129 * NOTE:
 130 *      ns_of_pid() is expected to be called for a process (task) that has
 131 *      an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
 132 *      is expected to be non-NULL. If @pid is NULL, caller should handle
 133 *      the resulting NULL pid-ns.
 134 */
 135static inline struct pid_namespace *ns_of_pid(struct pid *pid)
 136{
 137        struct pid_namespace *ns = NULL;
 138        if (pid)
 139                ns = pid->numbers[pid->level].ns;
 140        return ns;
 141}
 142
 143/*
 144 * is_child_reaper returns true if the pid is the init process
 145 * of the current namespace. As this one could be checked before
 146 * pid_ns->child_reaper is assigned in copy_process, we check
 147 * with the pid number.
 148 */
 149static inline bool is_child_reaper(struct pid *pid)
 150{
 151        return pid->numbers[pid->level].nr == 1;
 152}
 153
 154/*
 155 * the helpers to get the pid's id seen from different namespaces
 156 *
 157 * pid_nr()    : global id, i.e. the id seen from the init namespace;
 158 * pid_vnr()   : virtual id, i.e. the id seen from the pid namespace of
 159 *               current.
 160 * pid_nr_ns() : id seen from the ns specified.
 161 *
 162 * see also task_xid_nr() etc in include/linux/sched.h
 163 */
 164
 165static inline pid_t pid_nr(struct pid *pid)
 166{
 167        pid_t nr = 0;
 168        if (pid)
 169                nr = pid->numbers[0].nr;
 170        return nr;
 171}
 172
 173pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 174pid_t pid_vnr(struct pid *pid);
 175
 176#define do_each_pid_task(pid, type, task)                               \
 177        do {                                                            \
 178                struct hlist_node *pos___;                              \
 179                if ((pid) != NULL)                                      \
 180                        hlist_for_each_entry_rcu((task), pos___,        \
 181                                &(pid)->tasks[type], pids[type].node) {
 182
 183                        /*
 184                         * Both old and new leaders may be attached to
 185                         * the same pid in the middle of de_thread().
 186                         */
 187#define while_each_pid_task(pid, type, task)                            \
 188                                if (type == PIDTYPE_PID)                \
 189                                        break;                          \
 190                        }                                               \
 191        } while (0)
 192
 193#define do_each_pid_thread(pid, type, task)                             \
 194        do_each_pid_task(pid, type, task) {                             \
 195                struct task_struct *tg___ = task;                       \
 196                do {
 197
 198#define while_each_pid_thread(pid, type, task)                          \
 199                } while_each_thread(tg___, task);                       \
 200                task = tg___;                                           \
 201        } while_each_pid_task(pid, type, task)
 202#endif /* _LINUX_PID_H */
 203
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