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