linux/kernel/pid.c
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   1/*
   2 * Generic pidhash and scalable, time-bounded PID allocator
   3 *
   4 * (C) 2002-2003 William Irwin, IBM
   5 * (C) 2004 William Irwin, Oracle
   6 * (C) 2002-2004 Ingo Molnar, Red Hat
   7 *
   8 * pid-structures are backing objects for tasks sharing a given ID to chain
   9 * against. There is very little to them aside from hashing them and
  10 * parking tasks using given ID's on a list.
  11 *
  12 * The hash is always changed with the tasklist_lock write-acquired,
  13 * and the hash is only accessed with the tasklist_lock at least
  14 * read-acquired, so there's no additional SMP locking needed here.
  15 *
  16 * We have a list of bitmap pages, which bitmaps represent the PID space.
  17 * Allocating and freeing PIDs is completely lockless. The worst-case
  18 * allocation scenario when all but one out of 1 million PIDs possible are
  19 * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
  20 * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
  21 *
  22 * Pid namespaces:
  23 *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
  24 *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
  25 *     Many thanks to Oleg Nesterov for comments and help
  26 *
  27 */
  28
  29#include <linux/mm.h>
  30#include <linux/module.h>
  31#include <linux/slab.h>
  32#include <linux/init.h>
  33#include <linux/rculist.h>
  34#include <linux/bootmem.h>
  35#include <linux/hash.h>
  36#include <linux/pid_namespace.h>
  37#include <linux/init_task.h>
  38#include <linux/syscalls.h>
  39
  40#define pid_hashfn(nr, ns)      \
  41        hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
  42static struct hlist_head *pid_hash;
  43static unsigned int pidhash_shift = 4;
  44struct pid init_struct_pid = INIT_STRUCT_PID;
  45
  46int pid_max = PID_MAX_DEFAULT;
  47
  48#define RESERVED_PIDS           300
  49
  50int pid_max_min = RESERVED_PIDS + 1;
  51int pid_max_max = PID_MAX_LIMIT;
  52
  53#define BITS_PER_PAGE           (PAGE_SIZE*8)
  54#define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
  55
  56static inline int mk_pid(struct pid_namespace *pid_ns,
  57                struct pidmap *map, int off)
  58{
  59        return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
  60}
  61
  62#define find_next_offset(map, off)                                      \
  63                find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  64
  65/*
  66 * PID-map pages start out as NULL, they get allocated upon
  67 * first use and are never deallocated. This way a low pid_max
  68 * value does not cause lots of bitmaps to be allocated, but
  69 * the scheme scales to up to 4 million PIDs, runtime.
  70 */
  71struct pid_namespace init_pid_ns = {
  72        .kref = {
  73                .refcount       = ATOMIC_INIT(2),
  74        },
  75        .pidmap = {
  76                [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  77        },
  78        .last_pid = 0,
  79        .level = 0,
  80        .child_reaper = &init_task,
  81};
  82EXPORT_SYMBOL_GPL(init_pid_ns);
  83
  84int is_container_init(struct task_struct *tsk)
  85{
  86        int ret = 0;
  87        struct pid *pid;
  88
  89        rcu_read_lock();
  90        pid = task_pid(tsk);
  91        if (pid != NULL && pid->numbers[pid->level].nr == 1)
  92                ret = 1;
  93        rcu_read_unlock();
  94
  95        return ret;
  96}
  97EXPORT_SYMBOL(is_container_init);
  98
  99/*
 100 * Note: disable interrupts while the pidmap_lock is held as an
 101 * interrupt might come in and do read_lock(&tasklist_lock).
 102 *
 103 * If we don't disable interrupts there is a nasty deadlock between
 104 * detach_pid()->free_pid() and another cpu that does
 105 * spin_lock(&pidmap_lock) followed by an interrupt routine that does
 106 * read_lock(&tasklist_lock);
 107 *
 108 * After we clean up the tasklist_lock and know there are no
 109 * irq handlers that take it we can leave the interrupts enabled.
 110 * For now it is easier to be safe than to prove it can't happen.
 111 */
 112
 113static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 114
 115static void free_pidmap(struct upid *upid)
 116{
 117        int nr = upid->nr;
 118        struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 119        int offset = nr & BITS_PER_PAGE_MASK;
 120
 121        clear_bit(offset, map->page);
 122        atomic_inc(&map->nr_free);
 123}
 124
 125/*
 126 * If we started walking pids at 'base', is 'a' seen before 'b'?
 127 */
 128static int pid_before(int base, int a, int b)
 129{
 130        /*
 131         * This is the same as saying
 132         *
 133         * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 134         * and that mapping orders 'a' and 'b' with respect to 'base'.
 135         */
 136        return (unsigned)(a - base) < (unsigned)(b - base);
 137}
 138
 139/*
 140 * We might be racing with someone else trying to set pid_ns->last_pid.
 141 * We want the winner to have the "later" value, because if the
 142 * "earlier" value prevails, then a pid may get reused immediately.
 143 *
 144 * Since pids rollover, it is not sufficient to just pick the bigger
 145 * value.  We have to consider where we started counting from.
 146 *
 147 * 'base' is the value of pid_ns->last_pid that we observed when
 148 * we started looking for a pid.
 149 *
 150 * 'pid' is the pid that we eventually found.
 151 */
 152static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 153{
 154        int prev;
 155        int last_write = base;
 156        do {
 157                prev = last_write;
 158                last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 159        } while ((prev != last_write) && (pid_before(base, last_write, pid)));
 160}
 161
 162static int alloc_pidmap(struct pid_namespace *pid_ns)
 163{
 164        int i, offset, max_scan, pid, last = pid_ns->last_pid;
 165        struct pidmap *map;
 166
 167        pid = last + 1;
 168        if (pid >= pid_max)
 169                pid = RESERVED_PIDS;
 170        offset = pid & BITS_PER_PAGE_MASK;
 171        map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 172        /*
 173         * If last_pid points into the middle of the map->page we
 174         * want to scan this bitmap block twice, the second time
 175         * we start with offset == 0 (or RESERVED_PIDS).
 176         */
 177        max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 178        for (i = 0; i <= max_scan; ++i) {
 179                if (unlikely(!map->page)) {
 180                        void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 181                        /*
 182                         * Free the page if someone raced with us
 183                         * installing it:
 184                         */
 185                        spin_lock_irq(&pidmap_lock);
 186                        if (!map->page) {
 187                                map->page = page;
 188                                page = NULL;
 189                        }
 190                        spin_unlock_irq(&pidmap_lock);
 191                        kfree(page);
 192                        if (unlikely(!map->page))
 193                                break;
 194                }
 195                if (likely(atomic_read(&map->nr_free))) {
 196                        do {
 197                                if (!test_and_set_bit(offset, map->page)) {
 198                                        atomic_dec(&map->nr_free);
 199                                        set_last_pid(pid_ns, last, pid);
 200                                        return pid;
 201                                }
 202                                offset = find_next_offset(map, offset);
 203                                pid = mk_pid(pid_ns, map, offset);
 204                        } while (offset < BITS_PER_PAGE && pid < pid_max);
 205                }
 206                if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 207                        ++map;
 208                        offset = 0;
 209                } else {
 210                        map = &pid_ns->pidmap[0];
 211                        offset = RESERVED_PIDS;
 212                        if (unlikely(last == offset))
 213                                break;
 214                }
 215                pid = mk_pid(pid_ns, map, offset);
 216        }
 217        return -1;
 218}
 219
 220int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 221{
 222        int offset;
 223        struct pidmap *map, *end;
 224
 225        if (last >= PID_MAX_LIMIT)
 226                return -1;
 227
 228        offset = (last + 1) & BITS_PER_PAGE_MASK;
 229        map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 230        end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 231        for (; map < end; map++, offset = 0) {
 232                if (unlikely(!map->page))
 233                        continue;
 234                offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 235                if (offset < BITS_PER_PAGE)
 236                        return mk_pid(pid_ns, map, offset);
 237        }
 238        return -1;
 239}
 240
 241void put_pid(struct pid *pid)
 242{
 243        struct pid_namespace *ns;
 244
 245        if (!pid)
 246                return;
 247
 248        ns = pid->numbers[pid->level].ns;
 249        if ((atomic_read(&pid->count) == 1) ||
 250             atomic_dec_and_test(&pid->count)) {
 251                kmem_cache_free(ns->pid_cachep, pid);
 252                put_pid_ns(ns);
 253        }
 254}
 255EXPORT_SYMBOL_GPL(put_pid);
 256
 257static void delayed_put_pid(struct rcu_head *rhp)
 258{
 259        struct pid *pid = container_of(rhp, struct pid, rcu);
 260        put_pid(pid);
 261}
 262
 263void free_pid(struct pid *pid)
 264{
 265        /* We can be called with write_lock_irq(&tasklist_lock) held */
 266        int i;
 267        unsigned long flags;
 268
 269        spin_lock_irqsave(&pidmap_lock, flags);
 270        for (i = 0; i <= pid->level; i++)
 271                hlist_del_rcu(&pid->numbers[i].pid_chain);
 272        spin_unlock_irqrestore(&pidmap_lock, flags);
 273
 274        for (i = 0; i <= pid->level; i++)
 275                free_pidmap(pid->numbers + i);
 276
 277        call_rcu(&pid->rcu, delayed_put_pid);
 278}
 279
 280struct pid *alloc_pid(struct pid_namespace *ns)
 281{
 282        struct pid *pid;
 283        enum pid_type type;
 284        int i, nr;
 285        struct pid_namespace *tmp;
 286        struct upid *upid;
 287
 288        pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 289        if (!pid)
 290                goto out;
 291
 292        tmp = ns;
 293        for (i = ns->level; i >= 0; i--) {
 294                nr = alloc_pidmap(tmp);
 295                if (nr < 0)
 296                        goto out_free;
 297
 298                pid->numbers[i].nr = nr;
 299                pid->numbers[i].ns = tmp;
 300                tmp = tmp->parent;
 301        }
 302
 303        get_pid_ns(ns);
 304        pid->level = ns->level;
 305        atomic_set(&pid->count, 1);
 306        for (type = 0; type < PIDTYPE_MAX; ++type)
 307                INIT_HLIST_HEAD(&pid->tasks[type]);
 308
 309        upid = pid->numbers + ns->level;
 310        spin_lock_irq(&pidmap_lock);
 311        for ( ; upid >= pid->numbers; --upid)
 312                hlist_add_head_rcu(&upid->pid_chain,
 313                                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 314        spin_unlock_irq(&pidmap_lock);
 315
 316out:
 317        return pid;
 318
 319out_free:
 320        while (++i <= ns->level)
 321                free_pidmap(pid->numbers + i);
 322
 323        kmem_cache_free(ns->pid_cachep, pid);
 324        pid = NULL;
 325        goto out;
 326}
 327
 328struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 329{
 330        struct hlist_node *elem;
 331        struct upid *pnr;
 332
 333        hlist_for_each_entry_rcu(pnr, elem,
 334                        &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 335                if (pnr->nr == nr && pnr->ns == ns)
 336                        return container_of(pnr, struct pid,
 337                                        numbers[ns->level]);
 338
 339        return NULL;
 340}
 341EXPORT_SYMBOL_GPL(find_pid_ns);
 342
 343struct pid *find_vpid(int nr)
 344{
 345        return find_pid_ns(nr, current->nsproxy->pid_ns);
 346}
 347EXPORT_SYMBOL_GPL(find_vpid);
 348
 349/*
 350 * attach_pid() must be called with the tasklist_lock write-held.
 351 */
 352void attach_pid(struct task_struct *task, enum pid_type type,
 353                struct pid *pid)
 354{
 355        struct pid_link *link;
 356
 357        link = &task->pids[type];
 358        link->pid = pid;
 359        hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 360}
 361
 362static void __change_pid(struct task_struct *task, enum pid_type type,
 363                        struct pid *new)
 364{
 365        struct pid_link *link;
 366        struct pid *pid;
 367        int tmp;
 368
 369        link = &task->pids[type];
 370        pid = link->pid;
 371
 372        hlist_del_rcu(&link->node);
 373        link->pid = new;
 374
 375        for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 376                if (!hlist_empty(&pid->tasks[tmp]))
 377                        return;
 378
 379        free_pid(pid);
 380}
 381
 382void detach_pid(struct task_struct *task, enum pid_type type)
 383{
 384        __change_pid(task, type, NULL);
 385}
 386
 387void change_pid(struct task_struct *task, enum pid_type type,
 388                struct pid *pid)
 389{
 390        __change_pid(task, type, pid);
 391        attach_pid(task, type, pid);
 392}
 393
 394/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 395void transfer_pid(struct task_struct *old, struct task_struct *new,
 396                           enum pid_type type)
 397{
 398        new->pids[type].pid = old->pids[type].pid;
 399        hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 400}
 401
 402struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 403{
 404        struct task_struct *result = NULL;
 405        if (pid) {
 406                struct hlist_node *first;
 407                first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 408                                              lockdep_tasklist_lock_is_held());
 409                if (first)
 410                        result = hlist_entry(first, struct task_struct, pids[(type)].node);
 411        }
 412        return result;
 413}
 414EXPORT_SYMBOL(pid_task);
 415
 416/*
 417 * Must be called under rcu_read_lock().
 418 */
 419struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 420{
 421        rcu_lockdep_assert(rcu_read_lock_held());
 422        return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 423}
 424
 425struct task_struct *find_task_by_vpid(pid_t vnr)
 426{
 427        return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
 428}
 429
 430struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 431{
 432        struct pid *pid;
 433        rcu_read_lock();
 434        if (type != PIDTYPE_PID)
 435                task = task->group_leader;
 436        pid = get_pid(task->pids[type].pid);
 437        rcu_read_unlock();
 438        return pid;
 439}
 440EXPORT_SYMBOL_GPL(get_task_pid);
 441
 442struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 443{
 444        struct task_struct *result;
 445        rcu_read_lock();
 446        result = pid_task(pid, type);
 447        if (result)
 448                get_task_struct(result);
 449        rcu_read_unlock();
 450        return result;
 451}
 452EXPORT_SYMBOL_GPL(get_pid_task);
 453
 454struct pid *find_get_pid(pid_t nr)
 455{
 456        struct pid *pid;
 457
 458        rcu_read_lock();
 459        pid = get_pid(find_vpid(nr));
 460        rcu_read_unlock();
 461
 462        return pid;
 463}
 464EXPORT_SYMBOL_GPL(find_get_pid);
 465
 466pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 467{
 468        struct upid *upid;
 469        pid_t nr = 0;
 470
 471        if (pid && ns->level <= pid->level) {
 472                upid = &pid->numbers[ns->level];
 473                if (upid->ns == ns)
 474                        nr = upid->nr;
 475        }
 476        return nr;
 477}
 478
 479pid_t pid_vnr(struct pid *pid)
 480{
 481        return pid_nr_ns(pid, current->nsproxy->pid_ns);
 482}
 483EXPORT_SYMBOL_GPL(pid_vnr);
 484
 485pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 486                        struct pid_namespace *ns)
 487{
 488        pid_t nr = 0;
 489
 490        rcu_read_lock();
 491        if (!ns)
 492                ns = current->nsproxy->pid_ns;
 493        if (likely(pid_alive(task))) {
 494                if (type != PIDTYPE_PID)
 495                        task = task->group_leader;
 496                nr = pid_nr_ns(task->pids[type].pid, ns);
 497        }
 498        rcu_read_unlock();
 499
 500        return nr;
 501}
 502EXPORT_SYMBOL(__task_pid_nr_ns);
 503
 504pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 505{
 506        return pid_nr_ns(task_tgid(tsk), ns);
 507}
 508EXPORT_SYMBOL(task_tgid_nr_ns);
 509
 510struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 511{
 512        return ns_of_pid(task_pid(tsk));
 513}
 514EXPORT_SYMBOL_GPL(task_active_pid_ns);
 515
 516/*
 517 * Used by proc to find the first pid that is greater than or equal to nr.
 518 *
 519 * If there is a pid at nr this function is exactly the same as find_pid_ns.
 520 */
 521struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 522{
 523        struct pid *pid;
 524
 525        do {
 526                pid = find_pid_ns(nr, ns);
 527                if (pid)
 528                        break;
 529                nr = next_pidmap(ns, nr);
 530        } while (nr > 0);
 531
 532        return pid;
 533}
 534
 535/*
 536 * The pid hash table is scaled according to the amount of memory in the
 537 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 538 * more.
 539 */
 540void __init pidhash_init(void)
 541{
 542        int i, pidhash_size;
 543
 544        pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 545                                           HASH_EARLY | HASH_SMALL,
 546                                           &pidhash_shift, NULL, 4096);
 547        pidhash_size = 1 << pidhash_shift;
 548
 549        for (i = 0; i < pidhash_size; i++)
 550                INIT_HLIST_HEAD(&pid_hash[i]);
 551}
 552
 553void __init pidmap_init(void)
 554{
 555        /* bump default and minimum pid_max based on number of cpus */
 556        pid_max = min(pid_max_max, max_t(int, pid_max,
 557                                PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 558        pid_max_min = max_t(int, pid_max_min,
 559                                PIDS_PER_CPU_MIN * num_possible_cpus());
 560        pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 561
 562        init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 563        /* Reserve PID 0. We never call free_pidmap(0) */
 564        set_bit(0, init_pid_ns.pidmap[0].page);
 565        atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 566
 567        init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 568                        SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 569}
 570
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