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