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, int last)
 221{
 222        int offset;
 223        struct pidmap *map, *end;
 224
 225        offset = (last + 1) & BITS_PER_PAGE_MASK;
 226        map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 227        end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 228        for (; map < end; map++, offset = 0) {
 229                if (unlikely(!map->page))
 230                        continue;
 231                offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 232                if (offset < BITS_PER_PAGE)
 233                        return mk_pid(pid_ns, map, offset);
 234        }
 235        return -1;
 236}
 237
 238void put_pid(struct pid *pid)
 239{
 240        struct pid_namespace *ns;
 241
 242        if (!pid)
 243                return;
 244
 245        ns = pid->numbers[pid->level].ns;
 246        if ((atomic_read(&pid->count) == 1) ||
 247             atomic_dec_and_test(&pid->count)) {
 248                kmem_cache_free(ns->pid_cachep, pid);
 249                put_pid_ns(ns);
 250        }
 251}
 252EXPORT_SYMBOL_GPL(put_pid);
 253
 254static void delayed_put_pid(struct rcu_head *rhp)
 255{
 256        struct pid *pid = container_of(rhp, struct pid, rcu);
 257        put_pid(pid);
 258}
 259
 260void free_pid(struct pid *pid)
 261{
 262        /* We can be called with write_lock_irq(&tasklist_lock) held */
 263        int i;
 264        unsigned long flags;
 265
 266        spin_lock_irqsave(&pidmap_lock, flags);
 267        for (i = 0; i <= pid->level; i++)
 268                hlist_del_rcu(&pid->numbers[i].pid_chain);
 269        spin_unlock_irqrestore(&pidmap_lock, flags);
 270
 271        for (i = 0; i <= pid->level; i++)
 272                free_pidmap(pid->numbers + i);
 273
 274        call_rcu(&pid->rcu, delayed_put_pid);
 275}
 276
 277struct pid *alloc_pid(struct pid_namespace *ns)
 278{
 279        struct pid *pid;
 280        enum pid_type type;
 281        int i, nr;
 282        struct pid_namespace *tmp;
 283        struct upid *upid;
 284
 285        pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 286        if (!pid)
 287                goto out;
 288
 289        tmp = ns;
 290        for (i = ns->level; i >= 0; i--) {
 291                nr = alloc_pidmap(tmp);
 292                if (nr < 0)
 293                        goto out_free;
 294
 295                pid->numbers[i].nr = nr;
 296                pid->numbers[i].ns = tmp;
 297                tmp = tmp->parent;
 298        }
 299
 300        get_pid_ns(ns);
 301        pid->level = ns->level;
 302        atomic_set(&pid->count, 1);
 303        for (type = 0; type < PIDTYPE_MAX; ++type)
 304                INIT_HLIST_HEAD(&pid->tasks[type]);
 305
 306        upid = pid->numbers + ns->level;
 307        spin_lock_irq(&pidmap_lock);
 308        for ( ; upid >= pid->numbers; --upid)
 309                hlist_add_head_rcu(&upid->pid_chain,
 310                                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 311        spin_unlock_irq(&pidmap_lock);
 312
 313out:
 314        return pid;
 315
 316out_free:
 317        while (++i <= ns->level)
 318                free_pidmap(pid->numbers + i);
 319
 320        kmem_cache_free(ns->pid_cachep, pid);
 321        pid = NULL;
 322        goto out;
 323}
 324
 325struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 326{
 327        struct hlist_node *elem;
 328        struct upid *pnr;
 329
 330        hlist_for_each_entry_rcu(pnr, elem,
 331                        &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 332                if (pnr->nr == nr && pnr->ns == ns)
 333                        return container_of(pnr, struct pid,
 334                                        numbers[ns->level]);
 335
 336        return NULL;
 337}
 338EXPORT_SYMBOL_GPL(find_pid_ns);
 339
 340struct pid *find_vpid(int nr)
 341{
 342        return find_pid_ns(nr, current->nsproxy->pid_ns);
 343}
 344EXPORT_SYMBOL_GPL(find_vpid);
 345
 346/*
 347 * attach_pid() must be called with the tasklist_lock write-held.
 348 */
 349void attach_pid(struct task_struct *task, enum pid_type type,
 350                struct pid *pid)
 351{
 352        struct pid_link *link;
 353
 354        link = &task->pids[type];
 355        link->pid = pid;
 356        hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 357}
 358
 359static void __change_pid(struct task_struct *task, enum pid_type type,
 360                        struct pid *new)
 361{
 362        struct pid_link *link;
 363        struct pid *pid;
 364        int tmp;
 365
 366        link = &task->pids[type];
 367        pid = link->pid;
 368
 369        hlist_del_rcu(&link->node);
 370        link->pid = new;
 371
 372        for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 373                if (!hlist_empty(&pid->tasks[tmp]))
 374                        return;
 375
 376        free_pid(pid);
 377}
 378
 379void detach_pid(struct task_struct *task, enum pid_type type)
 380{
 381        __change_pid(task, type, NULL);
 382}
 383
 384void change_pid(struct task_struct *task, enum pid_type type,
 385                struct pid *pid)
 386{
 387        __change_pid(task, type, pid);
 388        attach_pid(task, type, pid);
 389}
 390
 391/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 392void transfer_pid(struct task_struct *old, struct task_struct *new,
 393                           enum pid_type type)
 394{
 395        new->pids[type].pid = old->pids[type].pid;
 396        hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 397}
 398
 399struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 400{
 401        struct task_struct *result = NULL;
 402        if (pid) {
 403                struct hlist_node *first;
 404                first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 405                                              rcu_read_lock_held() ||
 406                                              lockdep_tasklist_lock_is_held());
 407                if (first)
 408                        result = hlist_entry(first, struct task_struct, pids[(type)].node);
 409        }
 410        return result;
 411}
 412EXPORT_SYMBOL(pid_task);
 413
 414/*
 415 * Must be called under rcu_read_lock().
 416 */
 417struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 418{
 419        rcu_lockdep_assert(rcu_read_lock_held());
 420        return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 421}
 422
 423struct task_struct *find_task_by_vpid(pid_t vnr)
 424{
 425        return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
 426}
 427
 428struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 429{
 430        struct pid *pid;
 431        rcu_read_lock();
 432        if (type != PIDTYPE_PID)
 433                task = task->group_leader;
 434        pid = get_pid(task->pids[type].pid);
 435        rcu_read_unlock();
 436        return pid;
 437}
 438
 439struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 440{
 441        struct task_struct *result;
 442        rcu_read_lock();
 443        result = pid_task(pid, type);
 444        if (result)
 445                get_task_struct(result);
 446        rcu_read_unlock();
 447        return result;
 448}
 449
 450struct pid *find_get_pid(pid_t nr)
 451{
 452        struct pid *pid;
 453
 454        rcu_read_lock();
 455        pid = get_pid(find_vpid(nr));
 456        rcu_read_unlock();
 457
 458        return pid;
 459}
 460EXPORT_SYMBOL_GPL(find_get_pid);
 461
 462pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 463{
 464        struct upid *upid;
 465        pid_t nr = 0;
 466
 467        if (pid && ns->level <= pid->level) {
 468                upid = &pid->numbers[ns->level];
 469                if (upid->ns == ns)
 470                        nr = upid->nr;
 471        }
 472        return nr;
 473}
 474
 475pid_t pid_vnr(struct pid *pid)
 476{
 477        return pid_nr_ns(pid, current->nsproxy->pid_ns);
 478}
 479EXPORT_SYMBOL_GPL(pid_vnr);
 480
 481pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 482                        struct pid_namespace *ns)
 483{
 484        pid_t nr = 0;
 485
 486        rcu_read_lock();
 487        if (!ns)
 488                ns = current->nsproxy->pid_ns;
 489        if (likely(pid_alive(task))) {
 490                if (type != PIDTYPE_PID)
 491                        task = task->group_leader;
 492                nr = pid_nr_ns(task->pids[type].pid, ns);
 493        }
 494        rcu_read_unlock();
 495
 496        return nr;
 497}
 498EXPORT_SYMBOL(__task_pid_nr_ns);
 499
 500pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 501{
 502        return pid_nr_ns(task_tgid(tsk), ns);
 503}
 504EXPORT_SYMBOL(task_tgid_nr_ns);
 505
 506struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 507{
 508        return ns_of_pid(task_pid(tsk));
 509}
 510EXPORT_SYMBOL_GPL(task_active_pid_ns);
 511
 512/*
 513 * Used by proc to find the first pid that is greater than or equal to nr.
 514 *
 515 * If there is a pid at nr this function is exactly the same as find_pid_ns.
 516 */
 517struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 518{
 519        struct pid *pid;
 520
 521        do {
 522                pid = find_pid_ns(nr, ns);
 523                if (pid)
 524                        break;
 525                nr = next_pidmap(ns, nr);
 526        } while (nr > 0);
 527
 528        return pid;
 529}
 530
 531/*
 532 * The pid hash table is scaled according to the amount of memory in the
 533 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 534 * more.
 535 */
 536void __init pidhash_init(void)
 537{
 538        int i, pidhash_size;
 539
 540        pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 541                                           HASH_EARLY | HASH_SMALL,
 542                                           &pidhash_shift, NULL, 4096);
 543        pidhash_size = 1 << pidhash_shift;
 544
 545        for (i = 0; i < pidhash_size; i++)
 546                INIT_HLIST_HEAD(&pid_hash[i]);
 547}
 548
 549void __init pidmap_init(void)
 550{
 551        /* bump default and minimum pid_max based on number of cpus */
 552        pid_max = min(pid_max_max, max_t(int, pid_max,
 553                                PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 554        pid_max_min = max_t(int, pid_max_min,
 555                                PIDS_PER_CPU_MIN * num_possible_cpus());
 556        pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 557
 558        init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 559        /* Reserve PID 0. We never call free_pidmap(0) */
 560        set_bit(0, init_pid_ns.pidmap[0].page);
 561        atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 562
 563        init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 564                        SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 565}
 566
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