diff options
Diffstat (limited to 'include/linux/pid.h')
| -rw-r--r-- | include/linux/pid.h | 96 |
1 files changed, 81 insertions, 15 deletions
diff --git a/include/linux/pid.h b/include/linux/pid.h index 5b9082cc600..29960b03bef 100644 --- a/include/linux/pid.h +++ b/include/linux/pid.h @@ -1,6 +1,8 @@ #ifndef _LINUX_PID_H #define _LINUX_PID_H +#include <linux/rcupdate.h> + enum pid_type { PIDTYPE_PID, @@ -9,45 +11,109 @@ enum pid_type PIDTYPE_MAX }; +/* + * 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. + * + * 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. + * + * 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. + * + * 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 we don't mistakenly refer to new processes. + */ + struct pid { + atomic_t count; /* Try to keep pid_chain in the same cacheline as nr for find_pid */ int nr; struct hlist_node pid_chain; - /* list of pids with the same nr, only one of them is in the hash */ - struct list_head pid_list; + /* lists of tasks that use this pid */ + struct hlist_head tasks[PIDTYPE_MAX]; + struct rcu_head rcu; }; -#define pid_task(elem, type) \ - list_entry(elem, struct task_struct, pids[type].pid_list) +struct pid_link +{ + struct hlist_node node; + struct pid *pid; +}; + +static inline struct pid *get_pid(struct pid *pid) +{ + if (pid) + atomic_inc(&pid->count); + return pid; +} + +extern void FASTCALL(put_pid(struct pid *pid)); +extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type)); +extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid, + enum pid_type)); /* * attach_pid() and detach_pid() must be called with the tasklist_lock * write-held. */ -extern int FASTCALL(attach_pid(struct task_struct *task, enum pid_type type, int nr)); +extern int FASTCALL(attach_pid(struct task_struct *task, + enum pid_type type, int nr)); extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type)); /* * look up a PID in the hash table. Must be called with the tasklist_lock - * held. + * or rcu_read_lock() held. + */ +extern struct pid *FASTCALL(find_pid(int nr)); + +/* + * Lookup a PID in the hash table, and return with it's count elevated. */ -extern struct pid *FASTCALL(find_pid(enum pid_type, int)); +extern struct pid *find_get_pid(int nr); -extern int alloc_pidmap(void); -extern void FASTCALL(free_pidmap(int)); +extern struct pid *alloc_pid(void); +extern void FASTCALL(free_pid(struct pid *pid)); +#define pid_next(task, type) \ + ((task)->pids[(type)].node.next) + +#define pid_next_task(task, type) \ + hlist_entry(pid_next(task, type), struct task_struct, \ + pids[(type)].node) + + +/* We could use hlist_for_each_entry_rcu here but it takes more arguments + * than the do_each_task_pid/while_each_task_pid. So we roll our own + * to preserve the existing interface. + */ #define do_each_task_pid(who, type, task) \ if ((task = find_task_by_pid_type(type, who))) { \ - prefetch((task)->pids[type].pid_list.next); \ + prefetch(pid_next(task, type)); \ do { #define while_each_task_pid(who, type, task) \ - } while (task = pid_task((task)->pids[type].pid_list.next,\ - type), \ - prefetch((task)->pids[type].pid_list.next), \ - hlist_unhashed(&(task)->pids[type].pid_chain)); \ - } \ + } while (pid_next(task, type) && ({ \ + task = pid_next_task(task, type); \ + rcu_dereference(task); \ + prefetch(pid_next(task, type)); \ + 1; }) ); \ + } #endif /* _LINUX_PID_H */ |