c18258c6f0
In de_thread we move pids from one process to another, a rather ugly case. The function transfer_pid makes it clear what we are doing, and makes the action atomic. This is useful we ever want to atomically traverse the process group and session lists, in a rcu safe manner. Even if the atomic properties this change should be a win as transfer_pid should be less code to execute than executing both attach_pid and detach_pid, and this should make de_thread slightly smaller as only a single function call needs to be emitted. The only downside is that the code might be slower to execute as the odds are against transfer_pid being in cache. Signed-off-by: Eric W. Biederman <ebiederm@xmission.com> Cc: Oleg Nesterov <oleg@tv-sign.ru> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
121 lines
3.5 KiB
C
121 lines
3.5 KiB
C
#ifndef _LINUX_PID_H
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#define _LINUX_PID_H
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#include <linux/rcupdate.h>
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enum pid_type
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{
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PIDTYPE_PID,
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PIDTYPE_PGID,
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PIDTYPE_SID,
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PIDTYPE_MAX
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};
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/*
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* What is struct pid?
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*
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* A struct pid is the kernel's internal notion of a process identifier.
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* It refers to individual tasks, process groups, and sessions. While
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* there are processes attached to it the struct pid lives in a hash
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* table, so it and then the processes that it refers to can be found
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* quickly from the numeric pid value. The attached processes may be
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* quickly accessed by following pointers from struct pid.
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*
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* Storing pid_t values in the kernel and refering to them later has a
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* problem. The process originally with that pid may have exited and the
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* pid allocator wrapped, and another process could have come along
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* and been assigned that pid.
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*
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* Referring to user space processes by holding a reference to struct
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* task_struct has a problem. When the user space process exits
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* the now useless task_struct is still kept. A task_struct plus a
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* stack consumes around 10K of low kernel memory. More precisely
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* this is THREAD_SIZE + sizeof(struct task_struct). By comparison
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* a struct pid is about 64 bytes.
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*
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* Holding a reference to struct pid solves both of these problems.
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* It is small so holding a reference does not consume a lot of
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* resources, and since a new struct pid is allocated when the numeric
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* pid value is reused we don't mistakenly refer to new processes.
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*/
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struct pid
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{
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atomic_t count;
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/* Try to keep pid_chain in the same cacheline as nr for find_pid */
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int nr;
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struct hlist_node pid_chain;
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/* lists of tasks that use this pid */
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struct hlist_head tasks[PIDTYPE_MAX];
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struct rcu_head rcu;
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};
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struct pid_link
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{
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struct hlist_node node;
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struct pid *pid;
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};
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static inline struct pid *get_pid(struct pid *pid)
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{
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if (pid)
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atomic_inc(&pid->count);
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return pid;
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}
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extern void FASTCALL(put_pid(struct pid *pid));
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extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
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extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
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enum pid_type));
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/*
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* attach_pid() and detach_pid() must be called with the tasklist_lock
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* write-held.
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*/
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extern int FASTCALL(attach_pid(struct task_struct *task,
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enum pid_type type, int nr));
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extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
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extern void FASTCALL(transfer_pid(struct task_struct *old,
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struct task_struct *new, enum pid_type));
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/*
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* look up a PID in the hash table. Must be called with the tasklist_lock
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* or rcu_read_lock() held.
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*/
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extern struct pid *FASTCALL(find_pid(int nr));
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/*
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* Lookup a PID in the hash table, and return with it's count elevated.
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*/
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extern struct pid *find_get_pid(int nr);
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extern struct pid *alloc_pid(void);
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extern void FASTCALL(free_pid(struct pid *pid));
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#define pid_next(task, type) \
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((task)->pids[(type)].node.next)
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#define pid_next_task(task, type) \
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hlist_entry(pid_next(task, type), struct task_struct, \
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pids[(type)].node)
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/* We could use hlist_for_each_entry_rcu here but it takes more arguments
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* than the do_each_task_pid/while_each_task_pid. So we roll our own
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* to preserve the existing interface.
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*/
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#define do_each_task_pid(who, type, task) \
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if ((task = find_task_by_pid_type(type, who))) { \
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prefetch(pid_next(task, type)); \
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do {
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#define while_each_task_pid(who, type, task) \
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} while (pid_next(task, type) && ({ \
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task = pid_next_task(task, type); \
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rcu_dereference(task); \
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prefetch(pid_next(task, type)); \
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1; }) ); \
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}
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#endif /* _LINUX_PID_H */
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