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* Generic pidhash and scalable, time-bounded PID allocator
*
* (C) 2002 William Irwin, IBM
* (C) 2002 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 list_head *pid_hash[PIDTYPE_MAX];
static int pidhash_shift;
int pid_max = PID_MAX_DEFAULT;
int last_pid;
#define RESERVED_PIDS 300
#define PIDMAP_ENTRIES (PID_MAX_LIMIT/PAGE_SIZE/8)
#define BITS_PER_PAGE (PAGE_SIZE*8)
#define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)
/*
* 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 pidmap_t *map_limit = pidmap_array + PIDMAP_ENTRIES;
static spinlock_t pidmap_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
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);
}
/*
* Here we search for the next map that has free bits left.
* Normally the next map has free PIDs.
*/
static inline pidmap_t *next_free_map(pidmap_t *map, int *max_steps)
{
while (--*max_steps) {
if (++map == map_limit)
map = pidmap_array;
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 (!map->page)
break;
}
if (atomic_read(&map->nr_free))
return map;
}
return NULL;
}
int alloc_pidmap(void)
{
int pid, offset, max_steps = PIDMAP_ENTRIES + 1;
pidmap_t *map;
pid = last_pid + 1;
if (pid >= pid_max)
pid = RESERVED_PIDS;
offset = pid & BITS_PER_PAGE_MASK;
map = pidmap_array + pid / BITS_PER_PAGE;
if (likely(map->page && !test_and_set_bit(offset, map->page))) {
/*
* There is a small window for last_pid updates to race,
* but in that case the next allocation will go into the
* slowpath and that fixes things up.
*/
return_pid:
atomic_dec(&map->nr_free);
last_pid = pid;
return pid;
}
if (!offset || !atomic_read(&map->nr_free)) {
next_map:
map = next_free_map(map, &max_steps);
if (!map)
goto failure;
offset = 0;
}
/*
* Find the next zero bit:
*/
scan_more:
offset = find_next_zero_bit(map->page, BITS_PER_PAGE, offset);
if (offset >= BITS_PER_PAGE)
goto next_map;
if (test_and_set_bit(offset, map->page))
goto scan_more;
/* we got the PID: */
pid = (map - pidmap_array) * BITS_PER_PAGE + offset;
goto return_pid;
failure:
return -1;
}
fastcall struct pid *find_pid(enum pid_type type, int nr)
{
struct list_head *elem, *bucket = &pid_hash[type][pid_hashfn(nr)];
struct pid *pid;
__list_for_each(elem, bucket) {
pid = list_entry(elem, struct pid, hash_chain);
if (pid->nr == nr)
return pid;
}
return NULL;
}
void fastcall link_pid(task_t *task, struct pid_link *link, struct pid *pid)
{
atomic_inc(&pid->count);
list_add_tail(&link->pid_chain, &pid->task_list);
link->pidptr = pid;
}
int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
{
struct pid *pid = find_pid(type, nr);
if (pid)
atomic_inc(&pid->count);
else {
pid = &task->pids[type].pid;
pid->nr = nr;
atomic_set(&pid->count, 1);
INIT_LIST_HEAD(&pid->task_list);
pid->task = task;
get_task_struct(task);
list_add(&pid->hash_chain, &pid_hash[type][pid_hashfn(nr)]);
}
list_add_tail(&task->pids[type].pid_chain, &pid->task_list);
task->pids[type].pidptr = pid;
return 0;
}
static inline int __detach_pid(task_t *task, enum pid_type type)
{
struct pid_link *link = task->pids + type;
struct pid *pid = link->pidptr;
int nr;
list_del(&link->pid_chain);
if (!atomic_dec_and_test(&pid->count))
return 0;
nr = pid->nr;
list_del(&pid->hash_chain);
put_task_struct(pid->task);
return nr;
}
static void _detach_pid(task_t *task, enum pid_type type)
{
__detach_pid(task, type);
}
void fastcall detach_pid(task_t *task, enum pid_type type)
{
int nr = __detach_pid(task, type);
if (!nr)
return;
for (type = 0; type < PIDTYPE_MAX; ++type)
if (find_pid(type, nr))
return;
free_pidmap(nr);
}
task_t *find_task_by_pid(int nr)
{
struct pid *pid = find_pid(PIDTYPE_PID, nr);
if (!pid)
return NULL;
return pid_task(pid->task_list.next, PIDTYPE_PID);
}
EXPORT_SYMBOL(find_task_by_pid);
/*
* 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 = max_pfn >> (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,
pidhash_size * sizeof(struct list_head));
for (i = 0; i < PIDTYPE_MAX; i++) {
pid_hash[i] = alloc_bootmem(pidhash_size *
sizeof(struct list_head));
if (!pid_hash[i])
panic("Could not alloc pidhash!\n");
for (j = 0; j < pidhash_size; j++)
INIT_LIST_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);
}
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