/*
* 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_rcu(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);
task_pid->nr = nr;
if (pid == NULL) {
INIT_LIST_HEAD(&task_pid->pid_list);
hlist_add_head_rcu(&task_pid->pid_chain,
&pid_hash[type][pid_hashfn(nr)]);
} else {
INIT_HLIST_NODE(&task_pid->pid_chain);
list_add_tail_rcu(&task_pid->pid_list, &pid->pid_list);
}
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)) {
if (list_empty(&pid->pid_list)) {
nr = pid->nr;
hlist_del_rcu(&pid->pid_chain);
} else {
pid_next = list_entry(pid->pid_list.next,
struct pid, pid_list);
/* insert next pid from pid_list to hash */
hlist_replace_rcu(&pid->pid_chain,
&pid_next->pid_chain);
}
}
list_del_rcu(&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);
/*
* 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)
{
pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
/* Reserve PID 0. We never call free_pidmap(0) */
set_bit(0, pidmap_array->page);
atomic_dec(&pidmap_array->nr_free);
}
