kernel/pid.c - third_party/linux - Git at Google

 /*
  * Generic pidhash and scalable, time-bounded PID allocator
  *
  * (C) 2002-2003 William Irwin, IBM
  * (C) 2004 William Irwin, Oracle
  * (C) 2002-2004 Ingo Molnar, Red Hat
  *
  * pid-structures are backing objects for tasks sharing a given ID to chain
  * against. There is very little to them aside from hashing them and
  * parking tasks using given ID's on a list.
  *
  * The hash is always changed with the tasklist_lock write-acquired,
  * and the hash is only accessed with the tasklist_lock at least
  * read-acquired, so there's no additional SMP locking needed here.
  *
  * We have a list of bitmap pages, which bitmaps represent the PID space.
  * Allocating and freeing PIDs is completely lockless. The worst-case
  * allocation scenario when all but one out of 1 million PIDs possible are
  * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
  * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
  */

 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/bootmem.h>
 #include <linux/hash.h>

 #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
 static struct hlist_head *pid_hash[PIDTYPE_MAX];
 static int pidhash_shift;

 int pid_max = PID_MAX_DEFAULT;
 int last_pid;

 #define RESERVED_PIDS		300

 int pid_max_min = RESERVED_PIDS + 1;
 int pid_max_max = PID_MAX_LIMIT;

 #define PIDMAP_ENTRIES		((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8)
 #define BITS_PER_PAGE		(PAGE_SIZE*8)
 #define BITS_PER_PAGE_MASK	(BITS_PER_PAGE-1)
 #define mk_pid(map, off)	(((map) - pidmap_array)*BITS_PER_PAGE + (off))
 #define find_next_offset(map, off)					\
 		find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

 /*
  * PID-map pages start out as NULL, they get allocated upon
  * first use and are never deallocated. This way a low pid_max
  * value does not cause lots of bitmaps to be allocated, but
  * the scheme scales to up to 4 million PIDs, runtime.
  */
 typedef struct pidmap {
 	atomic_t nr_free;
 	void *page;
 } pidmap_t;

 static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
 	 { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };

 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

 fastcall void free_pidmap(int pid)
 {
 	pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
 	int offset = pid & BITS_PER_PAGE_MASK;

 	clear_bit(offset, map->page);
 	atomic_inc(&map->nr_free);
 }

 int alloc_pidmap(void)
 {
 	int i, offset, max_scan, pid, last = last_pid;
 	pidmap_t *map;

 	pid = last + 1;
 	if (pid >= pid_max)
 		pid = RESERVED_PIDS;
 	offset = pid & BITS_PER_PAGE_MASK;
 	map = &pidmap_array[pid/BITS_PER_PAGE];
 	max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
 	for (i = 0; i <= max_scan; ++i) {
 		if (unlikely(!map->page)) {
 			unsigned long page = get_zeroed_page(GFP_KERNEL);
 			/*
 			 * Free the page if someone raced with us
 			 * installing it:
 			 */
 			spin_lock(&pidmap_lock);
 			if (map->page)
 				free_page(page);
 			else
 				map->page = (void *)page;
 			spin_unlock(&pidmap_lock);
 			if (unlikely(!map->page))
 				break;
 		}
 		if (likely(atomic_read(&map->nr_free))) {
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
 					last_pid = pid;
 					return pid;
 				}
 				offset = find_next_offset(map, offset);
 				pid = mk_pid(map, offset);
 			/*
 			 * find_next_offset() found a bit, the pid from it
 			 * is in-bounds, and if we fell back to the last
 			 * bitmap block and the final block was the same
 			 * as the starting point, pid is before last_pid.
 			 */
 			} while (offset < BITS_PER_PAGE && pid < pid_max &&
 					(i != max_scan || pid < last ||
 					    !((last+1) & BITS_PER_PAGE_MASK)));
 		}
 		if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
 			++map;
 			offset = 0;
 		} else {
 			map = &pidmap_array[0];
 			offset = RESERVED_PIDS;
 			if (unlikely(last == offset))
 				break;
 		}
 		pid = mk_pid(map, offset);
 	}
 	return -1;
 }

 struct pid * fastcall find_pid(enum pid_type type, int nr)
 {
 	struct hlist_node *elem;
 	struct pid *pid;

 	hlist_for_each_entry(pid, elem,
 			&pid_hash[type][pid_hashfn(nr)], pid_chain) {
 		if (pid->nr == nr)
 			return pid;
 	}
 	return NULL;
 }

 int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
 {
 	struct pid *pid, *task_pid;

 	task_pid = &task->pids[type];
 	pid = find_pid(type, nr);
 	if (pid == NULL) {
 		hlist_add_head(&task_pid->pid_chain,
 				&pid_hash[type][pid_hashfn(nr)]);
 		INIT_LIST_HEAD(&task_pid->pid_list);
 	} else {
 		INIT_HLIST_NODE(&task_pid->pid_chain);
 		list_add_tail(&task_pid->pid_list, &pid->pid_list);
 	}
 	task_pid->nr = nr;

 	return 0;
 }

 static fastcall int __detach_pid(task_t *task, enum pid_type type)
 {
 	struct pid *pid, *pid_next;
 	int nr = 0;

 	pid = &task->pids[type];
 	if (!hlist_unhashed(&pid->pid_chain)) {
 		hlist_del(&pid->pid_chain);

 		if (list_empty(&pid->pid_list))
 			nr = pid->nr;
 		else {
 			pid_next = list_entry(pid->pid_list.next,
 						struct pid, pid_list);
 			/* insert next pid from pid_list to hash */
 			hlist_add_head(&pid_next->pid_chain,
 				&pid_hash[type][pid_hashfn(pid_next->nr)]);
 		}
 	}

 	list_del(&pid->pid_list);
 	pid->nr = 0;

 	return nr;
 }

 void fastcall detach_pid(task_t *task, enum pid_type type)
 {
 	int tmp, nr;

 	nr = __detach_pid(task, type);
 	if (!nr)
 		return;

 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 		if (tmp != type && find_pid(tmp, nr))
 			return;

 	free_pidmap(nr);
 }

 task_t *find_task_by_pid_type(int type, int nr)
 {
 	struct pid *pid;

 	pid = find_pid(type, nr);
 	if (!pid)
 		return NULL;

 	return pid_task(&pid->pid_list, type);
 }

 EXPORT_SYMBOL(find_task_by_pid_type);

 /*
  * This function switches the PIDs if a non-leader thread calls
  * sys_execve() - this must be done without releasing the PID.
  * (which a detach_pid() would eventually do.)
  */
 void switch_exec_pids(task_t *leader, task_t *thread)
 {
 	__detach_pid(leader, PIDTYPE_PID);
 	__detach_pid(leader, PIDTYPE_TGID);
 	__detach_pid(leader, PIDTYPE_PGID);
 	__detach_pid(leader, PIDTYPE_SID);

 	__detach_pid(thread, PIDTYPE_PID);
 	__detach_pid(thread, PIDTYPE_TGID);

 	leader->pid = leader->tgid = thread->pid;
 	thread->pid = thread->tgid;

 	attach_pid(thread, PIDTYPE_PID, thread->pid);
 	attach_pid(thread, PIDTYPE_TGID, thread->tgid);
 	attach_pid(thread, PIDTYPE_PGID, thread->signal->pgrp);
 	attach_pid(thread, PIDTYPE_SID, thread->signal->session);
 	list_add_tail(&thread->tasks, &init_task.tasks);

 	attach_pid(leader, PIDTYPE_PID, leader->pid);
 	attach_pid(leader, PIDTYPE_TGID, leader->tgid);
 	attach_pid(leader, PIDTYPE_PGID, leader->signal->pgrp);
 	attach_pid(leader, PIDTYPE_SID, leader->signal->session);
 }

 /*
  * The pid hash table is scaled according to the amount of memory in the
  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
  * more.
  */
 void __init pidhash_init(void)
 {
 	int i, j, pidhash_size;
 	unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);

 	pidhash_shift = max(4, fls(megabytes * 4));
 	pidhash_shift = min(12, pidhash_shift);
 	pidhash_size = 1 << pidhash_shift;

 	printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
 		pidhash_size, pidhash_shift,
 		PIDTYPE_MAX * pidhash_size * sizeof(struct hlist_head));

 	for (i = 0; i < PIDTYPE_MAX; i++) {
 		pid_hash[i] = alloc_bootmem(pidhash_size *
 					sizeof(*(pid_hash[i])));
 		if (!pid_hash[i])
 			panic("Could not alloc pidhash!\n");
 		for (j = 0; j < pidhash_size; j++)
 			INIT_HLIST_HEAD(&pid_hash[i][j]);
 	}
 }

 void __init pidmap_init(void)
 {
 	int i;

 	pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
 	set_bit(0, pidmap_array->page);
 	atomic_dec(&pidmap_array->nr_free);

 	/*
 	 * Allocate PID 0, and hash it via all PID types:
 	 */

 	for (i = 0; i < PIDTYPE_MAX; i++)
 		attach_pid(current, i, 0);
 }
	/*
	* Generic pidhash and scalable, time-bounded PID allocator
	*
	* (C) 2002-2003 William Irwin, IBM
	* (C) 2004 William Irwin, Oracle
	* (C) 2002-2004 Ingo Molnar, Red Hat
	*
	* pid-structures are backing objects for tasks sharing a given ID to chain
	* against. There is very little to them aside from hashing them and
	* parking tasks using given ID's on a list.
	*
	* The hash is always changed with the tasklist_lock write-acquired,
	* and the hash is only accessed with the tasklist_lock at least
	* read-acquired, so there's no additional SMP locking needed here.
	*
	* We have a list of bitmap pages, which bitmaps represent the PID space.
	* Allocating and freeing PIDs is completely lockless. The worst-case
	* allocation scenario when all but one out of 1 million PIDs possible are
	* allocated already: the scanning of 32 list entries and at most PAGE_SIZE
	* bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
	*/

	#include <linux/mm.h>
	#include <linux/module.h>
	#include <linux/slab.h>
	#include <linux/init.h>
	#include <linux/bootmem.h>
	#include <linux/hash.h>

	#define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
	static struct hlist_head *pid_hash[PIDTYPE_MAX];
	static int pidhash_shift;

	int pid_max = PID_MAX_DEFAULT;
	int last_pid;

	#define RESERVED_PIDS 300

	int pid_max_min = RESERVED_PIDS + 1;
	int pid_max_max = PID_MAX_LIMIT;

	#define PIDMAP_ENTRIES ((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8)
	#define BITS_PER_PAGE (PAGE_SIZE*8)
	#define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)
	#define mk_pid(map, off) (((map) - pidmap_array)*BITS_PER_PAGE + (off))
	#define find_next_offset(map, off) \
	find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

	/*
	* PID-map pages start out as NULL, they get allocated upon
	* first use and are never deallocated. This way a low pid_max
	* value does not cause lots of bitmaps to be allocated, but
	* the scheme scales to up to 4 million PIDs, runtime.
	*/
	typedef struct pidmap {
	atomic_t nr_free;
	void *page;
	} pidmap_t;

	static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
	{ [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };

	static __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

	fastcall void free_pidmap(int pid)
	{
	pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
	int offset = pid & BITS_PER_PAGE_MASK;

	clear_bit(offset, map->page);
	atomic_inc(&map->nr_free);
	}

	int alloc_pidmap(void)
	{
	int i, offset, max_scan, pid, last = last_pid;
	pidmap_t *map;

	pid = last + 1;
	if (pid >= pid_max)
	pid = RESERVED_PIDS;
	offset = pid & BITS_PER_PAGE_MASK;
	map = &pidmap_array[pid/BITS_PER_PAGE];
	max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
	for (i = 0; i <= max_scan; ++i) {
	if (unlikely(!map->page)) {
	unsigned long page = get_zeroed_page(GFP_KERNEL);
	/*
	* Free the page if someone raced with us
	* installing it:
	*/
	spin_lock(&pidmap_lock);
	if (map->page)
	free_page(page);
	else
	map->page = (void *)page;
	spin_unlock(&pidmap_lock);
	if (unlikely(!map->page))
	break;
	}
	if (likely(atomic_read(&map->nr_free))) {
	do {
	if (!test_and_set_bit(offset, map->page)) {
	atomic_dec(&map->nr_free);
	last_pid = pid;
	return pid;
	}
	offset = find_next_offset(map, offset);
	pid = mk_pid(map, offset);
	/*
	* find_next_offset() found a bit, the pid from it
	* is in-bounds, and if we fell back to the last
	* bitmap block and the final block was the same
	* as the starting point, pid is before last_pid.
	*/
	} while (offset < BITS_PER_PAGE && pid < pid_max &&
	(i != max_scan \|\| pid < last \|\|
	!((last+1) & BITS_PER_PAGE_MASK)));
	}
	if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
	++map;
	offset = 0;
	} else {
	map = &pidmap_array[0];
	offset = RESERVED_PIDS;
	if (unlikely(last == offset))
	break;
	}
	pid = mk_pid(map, offset);
	}
	return -1;
	}

	struct pid * fastcall find_pid(enum pid_type type, int nr)
	{
	struct hlist_node *elem;
	struct pid *pid;

	hlist_for_each_entry(pid, elem,
	&pid_hash[type][pid_hashfn(nr)], pid_chain) {
	if (pid->nr == nr)
	return pid;
	}
	return NULL;
	}

	int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
	{
	struct pid pid, task_pid;

	task_pid = &task->pids[type];
	pid = find_pid(type, nr);
	if (pid == NULL) {
	hlist_add_head(&task_pid->pid_chain,
	&pid_hash[type][pid_hashfn(nr)]);
	INIT_LIST_HEAD(&task_pid->pid_list);
	} else {
	INIT_HLIST_NODE(&task_pid->pid_chain);
	list_add_tail(&task_pid->pid_list, &pid->pid_list);
	}
	task_pid->nr = nr;

	return 0;
	}

	static fastcall int __detach_pid(task_t *task, enum pid_type type)
	{
	struct pid pid, pid_next;
	int nr = 0;

	pid = &task->pids[type];
	if (!hlist_unhashed(&pid->pid_chain)) {
	hlist_del(&pid->pid_chain);

	if (list_empty(&pid->pid_list))
	nr = pid->nr;
	else {
	pid_next = list_entry(pid->pid_list.next,
	struct pid, pid_list);
	/* insert next pid from pid_list to hash */
	hlist_add_head(&pid_next->pid_chain,
	&pid_hash[type][pid_hashfn(pid_next->nr)]);
	}
	}

	list_del(&pid->pid_list);
	pid->nr = 0;

	return nr;
	}

	void fastcall detach_pid(task_t *task, enum pid_type type)
	{
	int tmp, nr;

	nr = __detach_pid(task, type);
	if (!nr)
	return;

	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
	if (tmp != type && find_pid(tmp, nr))
	return;

	free_pidmap(nr);
	}

	task_t *find_task_by_pid_type(int type, int nr)
	{
	struct pid *pid;

	pid = find_pid(type, nr);
	if (!pid)
	return NULL;

	return pid_task(&pid->pid_list, type);
	}

	EXPORT_SYMBOL(find_task_by_pid_type);

	/*
	* This function switches the PIDs if a non-leader thread calls
	* sys_execve() - this must be done without releasing the PID.
	* (which a detach_pid() would eventually do.)
	*/
	void switch_exec_pids(task_t leader, task_t thread)
	{
	__detach_pid(leader, PIDTYPE_PID);
	__detach_pid(leader, PIDTYPE_TGID);
	__detach_pid(leader, PIDTYPE_PGID);
	__detach_pid(leader, PIDTYPE_SID);

	__detach_pid(thread, PIDTYPE_PID);
	__detach_pid(thread, PIDTYPE_TGID);

	leader->pid = leader->tgid = thread->pid;
	thread->pid = thread->tgid;

	attach_pid(thread, PIDTYPE_PID, thread->pid);
	attach_pid(thread, PIDTYPE_TGID, thread->tgid);
	attach_pid(thread, PIDTYPE_PGID, thread->signal->pgrp);
	attach_pid(thread, PIDTYPE_SID, thread->signal->session);
	list_add_tail(&thread->tasks, &init_task.tasks);

	attach_pid(leader, PIDTYPE_PID, leader->pid);
	attach_pid(leader, PIDTYPE_TGID, leader->tgid);
	attach_pid(leader, PIDTYPE_PGID, leader->signal->pgrp);
	attach_pid(leader, PIDTYPE_SID, leader->signal->session);
	}

	/*
	* The pid hash table is scaled according to the amount of memory in the
	* machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or
	* more.
	*/
	void __init pidhash_init(void)
	{
	int i, j, pidhash_size;
	unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);

	pidhash_shift = max(4, fls(megabytes * 4));
	pidhash_shift = min(12, pidhash_shift);
	pidhash_size = 1 << pidhash_shift;

	printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
	pidhash_size, pidhash_shift,
	PIDTYPE_MAX * pidhash_size * sizeof(struct hlist_head));

	for (i = 0; i < PIDTYPE_MAX; i++) {
	pid_hash[i] = alloc_bootmem(pidhash_size *
	sizeof(*(pid_hash[i])));
	if (!pid_hash[i])
	panic("Could not alloc pidhash!\n");
	for (j = 0; j < pidhash_size; j++)
	INIT_HLIST_HEAD(&pid_hash[i][j]);
	}
	}

	void __init pidmap_init(void)
	{
	int i;

	pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
	set_bit(0, pidmap_array->page);
	atomic_dec(&pidmap_array->nr_free);

	/*
	* Allocate PID 0, and hash it via all PID types:
	*/

	for (i = 0; i < PIDTYPE_MAX; i++)
	attach_pid(current, i, 0);
	}