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
  30 #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
  31 static struct hlist_head *pid_hash;
  32 static int pidhash_shift;
  33 static kmem_cache_t *pid_cachep;
  34
  35 int pid_max = PID_MAX_DEFAULT;
  36 int last_pid;
  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 PIDMAP_ENTRIES          ((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8)
  44 #define BITS_PER_PAGE           (PAGE_SIZE*8)
  45 #define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
  46 #define mk_pid(map, off)        (((map) - pidmap_array)*BITS_PER_PAGE + (off))
  47 #define find_next_offset(map, off)                                      \
  48                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  49
  50 /*
  51  * PID-map pages start out as NULL, they get allocated upon
  52  * first use and are never deallocated. This way a low pid_max
  53  * value does not cause lots of bitmaps to be allocated, but
  54  * the scheme scales to up to 4 million PIDs, runtime.
  55  */
  56 typedef struct pidmap {
  57         atomic_t nr_free;
  58         void *page;
  59 } pidmap_t;
  60
  61 static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
  62          { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };
  63
  64 /*
  65  * Note: disable interrupts while the pidmap_lock is held as an
  66  * interrupt might come in and do read_lock(&tasklist_lock).
  67  *
  68  * If we don't disable interrupts there is a nasty deadlock between
  69  * detach_pid()->free_pid() and another cpu that does
  70  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  71  * read_lock(&tasklist_lock);
  72  *
  73  * After we clean up the tasklist_lock and know there are no
  74  * irq handlers that take it we can leave the interrupts enabled.
  75  * For now it is easier to be safe than to prove it can't happen.
  76  */
  77 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
  78
  79 static fastcall void free_pidmap(int pid)
  80 {
  81         pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
  82         int offset = pid & BITS_PER_PAGE_MASK;
  83
  84         clear_bit(offset, map->page);
  85         atomic_inc(&map->nr_free);
  86 }
  87
  88 static int alloc_pidmap(void)
  89 {
  90         int i, offset, max_scan, pid, last = last_pid;
  91         pidmap_t *map;
  92
  93         pid = last + 1;
  94         if (pid >= pid_max)
  95                 pid = RESERVED_PIDS;
  96         offset = pid & BITS_PER_PAGE_MASK;
  97         map = &pidmap_array[pid/BITS_PER_PAGE];
  98         max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
  99         for (i = 0; i <= max_scan; ++i) {
 100                 if (unlikely(!map->page)) {
 101                         unsigned long page = get_zeroed_page(GFP_KERNEL);
 102                         /*
 103                          * Free the page if someone raced with us
 104                          * installing it:
 105                          */
 106                         spin_lock_irq(&pidmap_lock);
 107                         if (map->page)
 108                                 free_page(page);
 109                         else
 110                                 map->page = (void *)page;
 111                         spin_unlock_irq(&pidmap_lock);
 112                         if (unlikely(!map->page))
 113                                 break;
 114                 }
 115                 if (likely(atomic_read(&map->nr_free))) {
 116                         do {
 117                                 if (!test_and_set_bit(offset, map->page)) {
 118                                         atomic_dec(&map->nr_free);
 119                                         last_pid = pid;
 120                                         return pid;
 121                                 }
 122                                 offset = find_next_offset(map, offset);
 123                                 pid = mk_pid(map, offset);
 124                         /*
 125                          * find_next_offset() found a bit, the pid from it
 126                          * is in-bounds, and if we fell back to the last
 127                          * bitmap block and the final block was the same
 128                          * as the starting point, pid is before last_pid.
 129                          */
 130                         } while (offset < BITS_PER_PAGE && pid < pid_max &&
 131                                         (i != max_scan || pid < last ||
 132                                             !((last+1) & BITS_PER_PAGE_MASK)));
 133                 }
 134                 if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
 135                         ++map;
 136                         offset = 0;
 137                 } else {
 138                         map = &pidmap_array[0];
 139                         offset = RESERVED_PIDS;
 140                         if (unlikely(last == offset))
 141                                 break;
 142                 }
 143                 pid = mk_pid(map, offset);
 144         }
 145         return -1;
 146 }
 147
 148 fastcall void put_pid(struct pid *pid)
 149 {
 150         if (!pid)
 151                 return;
 152         if ((atomic_read(&pid->count) == 1) ||
 153              atomic_dec_and_test(&pid->count))
 154                 kmem_cache_free(pid_cachep, pid);
 155 }
 156
 157 static void delayed_put_pid(struct rcu_head *rhp)
 158 {
 159         struct pid *pid = container_of(rhp, struct pid, rcu);
 160         put_pid(pid);
 161 }
 162
 163 fastcall void free_pid(struct pid *pid)
 164 {
 165         /* We can be called with write_lock_irq(&tasklist_lock) held */
 166         unsigned long flags;
 167
 168         spin_lock_irqsave(&pidmap_lock, flags);
 169         hlist_del_rcu(&pid->pid_chain);
 170         spin_unlock_irqrestore(&pidmap_lock, flags);
 171
 172         free_pidmap(pid->nr);
 173         call_rcu(&pid->rcu, delayed_put_pid);
 174 }
 175
 176 struct pid *alloc_pid(void)
 177 {
 178         struct pid *pid;
 179         enum pid_type type;
 180         int nr = -1;
 181
 182         pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
 183         if (!pid)
 184                 goto out;
 185
 186         nr = alloc_pidmap();
 187         if (nr < 0)
 188                 goto out_free;
 189
 190         atomic_set(&pid->count, 1);
 191         pid->nr = nr;
 192         for (type = 0; type < PIDTYPE_MAX; ++type)
 193                 INIT_HLIST_HEAD(&pid->tasks[type]);
 194
 195         spin_lock_irq(&pidmap_lock);
 196         hlist_add_head_rcu(&pid->pid_chain, &pid_hash[pid_hashfn(pid->nr)]);
 197         spin_unlock_irq(&pidmap_lock);
 198
 199 out:
 200         return pid;
 201
 202 out_free:
 203         kmem_cache_free(pid_cachep, pid);
 204         pid = NULL;
 205         goto out;
 206 }
 207
 208 struct pid * fastcall find_pid(int nr)
 209 {
 210         struct hlist_node *elem;
 211         struct pid *pid;
 212
 213         hlist_for_each_entry_rcu(pid, elem,
 214                         &pid_hash[pid_hashfn(nr)], pid_chain) {
 215                 if (pid->nr == nr)
 216                         return pid;
 217         }
 218         return NULL;
 219 }
 220
 221 int fastcall attach_pid(struct task_struct *task, enum pid_type type, int nr)
 222 {
 223         struct pid_link *link;
 224         struct pid *pid;
 225
 226         WARN_ON(!task->pid); /* to be removed soon */
 227         WARN_ON(!nr); /* to be removed soon */
 228
 229         link = &task->pids[type];
 230         link->pid = pid = find_pid(nr);
 231         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 232
 233         return 0;
 234 }
 235
 236 void fastcall detach_pid(struct task_struct *task, enum pid_type type)
 237 {
 238         struct pid_link *link;
 239         struct pid *pid;
 240         int tmp;
 241
 242         link = &task->pids[type];
 243         pid = link->pid;
 244
 245         hlist_del_rcu(&link->node);
 246         link->pid = NULL;
 247
 248         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 249                 if (!hlist_empty(&pid->tasks[tmp]))
 250                         return;
 251
 252         free_pid(pid);
 253 }
 254
 255 struct task_struct * fastcall pid_task(struct pid *pid, enum pid_type type)
 256 {
 257         struct task_struct *result = NULL;
 258         if (pid) {
 259                 struct hlist_node *first;
 260                 first = rcu_dereference(pid->tasks[type].first);
 261                 if (first)
 262                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 263         }
 264         return result;
 265 }
 266
 267 /*
 268  * Must be called under rcu_read_lock() or with tasklist_lock read-held.
 269  */
 270 struct task_struct *find_task_by_pid_type(int type, int nr)
 271 {
 272         return pid_task(find_pid(nr), type);
 273 }
 274
 275 EXPORT_SYMBOL(find_task_by_pid_type);
 276
 277 struct task_struct *fastcall get_pid_task(struct pid *pid, enum pid_type type)
 278 {
 279         struct task_struct *result;
 280         rcu_read_lock();
 281         result = pid_task(pid, type);
 282         if (result)
 283                 get_task_struct(result);
 284         rcu_read_unlock();
 285         return result;
 286 }
 287
 288 struct pid *find_get_pid(pid_t nr)
 289 {
 290         struct pid *pid;
 291
 292         rcu_read_lock();
 293         pid = get_pid(find_pid(nr));
 294         rcu_read_unlock();
 295
 296         return pid;
 297 }
 298
 299 /*
 300  * The pid hash table is scaled according to the amount of memory in the
 301  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 302  * more.
 303  */
 304 void __init pidhash_init(void)
 305 {
 306         int i, pidhash_size;
 307         unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
 308
 309         pidhash_shift = max(4, fls(megabytes * 4));
 310         pidhash_shift = min(12, pidhash_shift);
 311         pidhash_size = 1 << pidhash_shift;
 312
 313         printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
 314                 pidhash_size, pidhash_shift,
 315                 pidhash_size * sizeof(struct hlist_head));
 316
 317         pid_hash = alloc_bootmem(pidhash_size * sizeof(*(pid_hash)));
 318         if (!pid_hash)
 319                 panic("Could not alloc pidhash!\n");
 320         for (i = 0; i < pidhash_size; i++)
 321                 INIT_HLIST_HEAD(&pid_hash[i]);
 322 }
 323
 324 void __init pidmap_init(void)
 325 {
 326         pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
 327         /* Reserve PID 0. We never call free_pidmap(0) */
 328         set_bit(0, pidmap_array->page);
 329         atomic_dec(&pidmap_array->nr_free);
 330
 331         pid_cachep = kmem_cache_create("pid", sizeof(struct pid),
 332                                         __alignof__(struct pid),
 333                                         SLAB_PANIC, NULL, NULL);
 334 }