kernel/pid.c

   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
  23 #include <linux/mm.h>
  24 #include <linux/module.h>
  25 #include <linux/slab.h>
  26 #include <linux/init.h>
  27 #include <linux/bootmem.h>
  28 #include <linux/hash.h>
  29 #include <linux/pid_namespace.h>
  30
  31 #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
  32 static struct hlist_head *pid_hash;
  33 static int pidhash_shift;
  34 static struct kmem_cache *pid_cachep;
  35
  36 int pid_max = PID_MAX_DEFAULT;
  37
  38 #define RESERVED_PIDS           300
  39
  40 int pid_max_min = RESERVED_PIDS + 1;
  41 int pid_max_max = PID_MAX_LIMIT;
  42
  43 #define BITS_PER_PAGE           (PAGE_SIZE*8)
  44 #define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
  45
  46 static inline int mk_pid(struct pid_namespace *pid_ns,
  47                 struct pidmap *map, int off)
  48 {
  49         return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
  50 }
  51
  52 #define find_next_offset(map, off)                                      \
  53                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  54
  55 /*
  56  * PID-map pages start out as NULL, they get allocated upon
  57  * first use and are never deallocated. This way a low pid_max
  58  * value does not cause lots of bitmaps to be allocated, but
  59  * the scheme scales to up to 4 million PIDs, runtime.
  60  */
  61 struct pid_namespace init_pid_ns = {
  62         .kref = {
  63                 .refcount       = ATOMIC_INIT(2),
  64         },
  65         .pidmap = {
  66                 [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  67         },
  68         .last_pid = 0,
  69         .child_reaper = &init_task
  70 };
  71
  72 /*
  73  * Note: disable interrupts while the pidmap_lock is held as an
  74  * interrupt might come in and do read_lock(&tasklist_lock).
  75  *
  76  * If we don't disable interrupts there is a nasty deadlock between
  77  * detach_pid()->free_pid() and another cpu that does
  78  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  79  * read_lock(&tasklist_lock);
  80  *
  81  * After we clean up the tasklist_lock and know there are no
  82  * irq handlers that take it we can leave the interrupts enabled.
  83  * For now it is easier to be safe than to prove it can't happen.
  84  */
  85
  86 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
  87
  88 static fastcall void free_pidmap(struct pid_namespace *pid_ns, int pid)
  89 {
  90         struct pidmap *map = pid_ns->pidmap + pid / BITS_PER_PAGE;
  91         int offset = pid & BITS_PER_PAGE_MASK;
  92
  93         clear_bit(offset, map->page);
  94         atomic_inc(&map->nr_free);
  95 }
  96
  97 static int alloc_pidmap(struct pid_namespace *pid_ns)
  98 {
  99         int i, offset, max_scan, pid, last = pid_ns->last_pid;
 100         struct pidmap *map;
 101
 102         pid = last + 1;
 103         if (pid >= pid_max)
 104                 pid = RESERVED_PIDS;
 105         offset = pid & BITS_PER_PAGE_MASK;
 106         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 107         max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
 108         for (i = 0; i <= max_scan; ++i) {
 109                 if (unlikely(!map->page)) {
 110                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 111                         /*
 112                          * Free the page if someone raced with us
 113                          * installing it:
 114                          */
 115                         spin_lock_irq(&pidmap_lock);
 116                         if (map->page)
 117                                 kfree(page);
 118                         else
 119                                 map->page = page;
 120                         spin_unlock_irq(&pidmap_lock);
 121                         if (unlikely(!map->page))
 122                                 break;
 123                 }
 124                 if (likely(atomic_read(&map->nr_free))) {
 125                         do {
 126                                 if (!test_and_set_bit(offset, map->page)) {
 127                                         atomic_dec(&map->nr_free);
 128                                         pid_ns->last_pid = pid;
 129                                         return pid;
 130                                 }
 131                                 offset = find_next_offset(map, offset);
 132                                 pid = mk_pid(pid_ns, map, offset);
 133                         /*
 134                          * find_next_offset() found a bit, the pid from it
 135                          * is in-bounds, and if we fell back to the last
 136                          * bitmap block and the final block was the same
 137                          * as the starting point, pid is before last_pid.
 138                          */
 139                         } while (offset < BITS_PER_PAGE && pid < pid_max &&
 140                                         (i != max_scan || pid < last ||
 141                                             !((last+1) & BITS_PER_PAGE_MASK)));
 142                 }
 143                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 144                         ++map;
 145                         offset = 0;
 146                 } else {
 147                         map = &pid_ns->pidmap[0];
 148                         offset = RESERVED_PIDS;
 149                         if (unlikely(last == offset))
 150                                 break;
 151                 }
 152                 pid = mk_pid(pid_ns, map, offset);
 153         }
 154         return -1;
 155 }
 156
 157 static int next_pidmap(struct pid_namespace *pid_ns, int last)
 158 {
 159         int offset;
 160         struct pidmap *map, *end;
 161
 162         offset = (last + 1) & BITS_PER_PAGE_MASK;
 163         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 164         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 165         for (; map < end; map++, offset = 0) {
 166                 if (unlikely(!map->page))
 167                         continue;
 168                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 169                 if (offset < BITS_PER_PAGE)
 170                         return mk_pid(pid_ns, map, offset);
 171         }
 172         return -1;
 173 }
 174
 175 fastcall void put_pid(struct pid *pid)
 176 {
 177         if (!pid)
 178                 return;
 179         if ((atomic_read(&pid->count) == 1) ||
 180              atomic_dec_and_test(&pid->count))
 181                 kmem_cache_free(pid_cachep, pid);
 182 }
 183 EXPORT_SYMBOL_GPL(put_pid);
 184
 185 static void delayed_put_pid(struct rcu_head *rhp)
 186 {
 187         struct pid *pid = container_of(rhp, struct pid, rcu);
 188         put_pid(pid);
 189 }
 190
 191 fastcall void free_pid(struct pid *pid)
 192 {
 193         /* We can be called with write_lock_irq(&tasklist_lock) held */
 194         unsigned long flags;
 195
 196         spin_lock_irqsave(&pidmap_lock, flags);
 197         hlist_del_rcu(&pid->pid_chain);
 198         spin_unlock_irqrestore(&pidmap_lock, flags);
 199
 200         free_pidmap(&init_pid_ns, pid->nr);
 201         call_rcu(&pid->rcu, delayed_put_pid);
 202 }
 203
 204 struct pid *alloc_pid(void)
 205 {
 206         struct pid *pid;
 207         enum pid_type type;
 208         int nr = -1;
 209
 210         pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
 211         if (!pid)
 212                 goto out;
 213
 214         nr = alloc_pidmap(current->nsproxy->pid_ns);
 215         if (nr < 0)
 216                 goto out_free;
 217
 218         atomic_set(&pid->count, 1);
 219         pid->nr = nr;
 220         for (type = 0; type < PIDTYPE_MAX; ++type)
 221                 INIT_HLIST_HEAD(&pid->tasks[type]);
 222
 223         spin_lock_irq(&pidmap_lock);
 224         hlist_add_head_rcu(&pid->pid_chain, &pid_hash[pid_hashfn(pid->nr)]);
 225         spin_unlock_irq(&pidmap_lock);
 226
 227 out:
 228         return pid;
 229
 230 out_free:
 231         kmem_cache_free(pid_cachep, pid);
 232         pid = NULL;
 233         goto out;
 234 }
 235
 236 struct pid * fastcall find_pid(int nr)
 237 {
 238         struct hlist_node *elem;
 239         struct pid *pid;
 240
 241         hlist_for_each_entry_rcu(pid, elem,
 242                         &pid_hash[pid_hashfn(nr)], pid_chain) {
 243                 if (pid->nr == nr)
 244                         return pid;
 245         }
 246         return NULL;
 247 }
 248 EXPORT_SYMBOL_GPL(find_pid);
 249
 250 int fastcall attach_pid(struct task_struct *task, enum pid_type type, int nr)
 251 {
 252         struct pid_link *link;
 253         struct pid *pid;
 254
 255         link = &task->pids[type];
 256         link->pid = pid = find_pid(nr);
 257         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 258
 259         return 0;
 260 }
 261
 262 void fastcall detach_pid(struct task_struct *task, enum pid_type type)
 263 {
 264         struct pid_link *link;
 265         struct pid *pid;
 266         int tmp;
 267
 268         link = &task->pids[type];
 269         pid = link->pid;
 270
 271         hlist_del_rcu(&link->node);
 272         link->pid = NULL;
 273
 274         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 275                 if (!hlist_empty(&pid->tasks[tmp]))
 276                         return;
 277
 278         free_pid(pid);
 279 }
 280
 281 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 282 void fastcall transfer_pid(struct task_struct *old, struct task_struct *new,
 283                            enum pid_type type)
 284 {
 285         new->pids[type].pid = old->pids[type].pid;
 286         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 287         old->pids[type].pid = NULL;
 288 }
 289
 290 struct task_struct * fastcall pid_task(struct pid *pid, enum pid_type type)
 291 {
 292         struct task_struct *result = NULL;
 293         if (pid) {
 294                 struct hlist_node *first;
 295                 first = rcu_dereference(pid->tasks[type].first);
 296                 if (first)
 297                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 298         }
 299         return result;
 300 }
 301
 302 /*
 303  * Must be called under rcu_read_lock() or with tasklist_lock read-held.
 304  */
 305 struct task_struct *find_task_by_pid_type(int type, int nr)
 306 {
 307         return pid_task(find_pid(nr), type);
 308 }
 309
 310 EXPORT_SYMBOL(find_task_by_pid_type);
 311
 312 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 313 {
 314         struct pid *pid;
 315         rcu_read_lock();
 316         pid = get_pid(task->pids[type].pid);
 317         rcu_read_unlock();
 318         return pid;
 319 }
 320
 321 struct task_struct *fastcall get_pid_task(struct pid *pid, enum pid_type type)
 322 {
 323         struct task_struct *result;
 324         rcu_read_lock();
 325         result = pid_task(pid, type);
 326         if (result)
 327                 get_task_struct(result);
 328         rcu_read_unlock();
 329         return result;
 330 }
 331
 332 struct pid *find_get_pid(pid_t nr)
 333 {
 334         struct pid *pid;
 335
 336         rcu_read_lock();
 337         pid = get_pid(find_pid(nr));
 338         rcu_read_unlock();
 339
 340         return pid;
 341 }
 342
 343 /*
 344  * Used by proc to find the first pid that is greater then or equal to nr.
 345  *
 346  * If there is a pid at nr this function is exactly the same as find_pid.
 347  */
 348 struct pid *find_ge_pid(int nr)
 349 {
 350         struct pid *pid;
 351
 352         do {
 353                 pid = find_pid(nr);
 354                 if (pid)
 355                         break;
 356                 nr = next_pidmap(current->nsproxy->pid_ns, nr);
 357         } while (nr > 0);
 358
 359         return pid;
 360 }
 361 EXPORT_SYMBOL_GPL(find_get_pid);
 362
 363 struct pid_namespace *copy_pid_ns(int flags, struct pid_namespace *old_ns)
 364 {
 365         BUG_ON(!old_ns);
 366         get_pid_ns(old_ns);
 367         return old_ns;
 368 }
 369
 370 void free_pid_ns(struct kref *kref)
 371 {
 372         struct pid_namespace *ns;
 373
 374         ns = container_of(kref, struct pid_namespace, kref);
 375         kfree(ns);
 376 }
 377
 378 /*
 379  * The pid hash table is scaled according to the amount of memory in the
 380  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 381  * more.
 382  */
 383 void __init pidhash_init(void)
 384 {
 385         int i, pidhash_size;
 386         unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
 387
 388         pidhash_shift = max(4, fls(megabytes * 4));
 389         pidhash_shift = min(12, pidhash_shift);
 390         pidhash_size = 1 << pidhash_shift;
 391
 392         printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
 393                 pidhash_size, pidhash_shift,
 394                 pidhash_size * sizeof(struct hlist_head));
 395
 396         pid_hash = alloc_bootmem(pidhash_size * sizeof(*(pid_hash)));
 397         if (!pid_hash)
 398                 panic("Could not alloc pidhash!\n");
 399         for (i = 0; i < pidhash_size; i++)
 400                 INIT_HLIST_HEAD(&pid_hash[i]);
 401 }
 402
 403 void __init pidmap_init(void)
 404 {
 405         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 406         /* Reserve PID 0. We never call free_pidmap(0) */
 407         set_bit(0, init_pid_ns.pidmap[0].page);
 408         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 409
 410         pid_cachep = KMEM_CACHE(pid, SLAB_PANIC);
 411 }