2 * Generic pidhash and scalable, time-bounded PID allocator
4 * (C) 2002-2003 William Irwin, IBM
5 * (C) 2004 William Irwin, Oracle
6 * (C) 2002-2004 Ingo Molnar, Red Hat
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.
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.
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).
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/vs_cvirt.h>
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;
36 int pid_max = PID_MAX_DEFAULT;
39 #define RESERVED_PIDS 300
41 int pid_max_min = RESERVED_PIDS + 1;
42 int pid_max_max = PID_MAX_LIMIT;
44 #define PIDMAP_ENTRIES ((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8)
45 #define BITS_PER_PAGE (PAGE_SIZE*8)
46 #define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)
47 #define mk_pid(map, off) (((map) - pidmap_array)*BITS_PER_PAGE + (off))
48 #define find_next_offset(map, off) \
49 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
52 * PID-map pages start out as NULL, they get allocated upon
53 * first use and are never deallocated. This way a low pid_max
54 * value does not cause lots of bitmaps to be allocated, but
55 * the scheme scales to up to 4 million PIDs, runtime.
57 typedef struct pidmap {
62 static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
63 { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };
66 * Note: disable interrupts while the pidmap_lock is held as an
67 * interrupt might come in and do read_lock(&tasklist_lock).
69 * If we don't disable interrupts there is a nasty deadlock between
70 * detach_pid()->free_pid() and another cpu that does
71 * spin_lock(&pidmap_lock) followed by an interrupt routine that does
72 * read_lock(&tasklist_lock);
74 * After we clean up the tasklist_lock and know there are no
75 * irq handlers that take it we can leave the interrupts enabled.
76 * For now it is easier to be safe than to prove it can't happen.
78 static __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
80 static fastcall void free_pidmap(int pid)
82 pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
83 int offset = pid & BITS_PER_PAGE_MASK;
85 clear_bit(offset, map->page);
86 atomic_inc(&map->nr_free);
89 static int alloc_pidmap(void)
91 int i, offset, max_scan, pid, last = last_pid;
97 offset = pid & BITS_PER_PAGE_MASK;
98 map = &pidmap_array[pid/BITS_PER_PAGE];
99 max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
100 for (i = 0; i <= max_scan; ++i) {
101 if (unlikely(!map->page)) {
102 unsigned long page = get_zeroed_page(GFP_KERNEL);
104 * Free the page if someone raced with us
107 spin_lock_irq(&pidmap_lock);
111 map->page = (void *)page;
112 spin_unlock_irq(&pidmap_lock);
113 if (unlikely(!map->page))
116 if (likely(atomic_read(&map->nr_free))) {
118 if (!test_and_set_bit(offset, map->page)) {
119 atomic_dec(&map->nr_free);
123 offset = find_next_offset(map, offset);
124 pid = mk_pid(map, offset);
126 * find_next_offset() found a bit, the pid from it
127 * is in-bounds, and if we fell back to the last
128 * bitmap block and the final block was the same
129 * as the starting point, pid is before last_pid.
131 } while (offset < BITS_PER_PAGE && pid < pid_max &&
132 (i != max_scan || pid < last ||
133 !((last+1) & BITS_PER_PAGE_MASK)));
135 if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
139 map = &pidmap_array[0];
140 offset = RESERVED_PIDS;
141 if (unlikely(last == offset))
144 pid = mk_pid(map, offset);
149 fastcall void put_pid(struct pid *pid)
153 if ((atomic_read(&pid->count) == 1) ||
154 atomic_dec_and_test(&pid->count))
155 kmem_cache_free(pid_cachep, pid);
158 static void delayed_put_pid(struct rcu_head *rhp)
160 struct pid *pid = container_of(rhp, struct pid, rcu);
164 fastcall void free_pid(struct pid *pid)
166 /* We can be called with write_lock_irq(&tasklist_lock) held */
169 spin_lock_irqsave(&pidmap_lock, flags);
170 hlist_del_rcu(&pid->pid_chain);
171 spin_unlock_irqrestore(&pidmap_lock, flags);
173 free_pidmap(pid->nr);
174 call_rcu(&pid->rcu, delayed_put_pid);
177 struct pid *alloc_pid(void)
183 pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
191 atomic_set(&pid->count, 1);
193 for (type = 0; type < PIDTYPE_MAX; ++type)
194 INIT_HLIST_HEAD(&pid->tasks[type]);
196 spin_lock_irq(&pidmap_lock);
197 hlist_add_head_rcu(&pid->pid_chain, &pid_hash[pid_hashfn(pid->nr)]);
198 spin_unlock_irq(&pidmap_lock);
204 kmem_cache_free(pid_cachep, pid);
209 struct pid * fastcall find_pid(int nr)
211 struct hlist_node *elem;
214 hlist_for_each_entry_rcu(pid, elem,
215 &pid_hash[pid_hashfn(nr)], pid_chain) {
222 int fastcall attach_pid(struct task_struct *task, enum pid_type type, int nr)
224 struct pid_link *link;
227 WARN_ON(!task->pid); /* to be removed soon */
228 WARN_ON(!nr); /* to be removed soon */
230 link = &task->pids[type];
231 link->pid = pid = find_pid(nr);
232 hlist_add_head_rcu(&link->node, &pid->tasks[type]);
237 void fastcall detach_pid(struct task_struct *task, enum pid_type type)
239 struct pid_link *link;
243 link = &task->pids[type];
246 hlist_del_rcu(&link->node);
249 for (tmp = PIDTYPE_MAX; --tmp >= 0; )
250 if (!hlist_empty(&pid->tasks[tmp]))
256 struct task_struct * fastcall pid_task(struct pid *pid, enum pid_type type)
258 struct task_struct *result = NULL;
260 struct hlist_node *first;
261 first = rcu_dereference(pid->tasks[type].first);
263 result = hlist_entry(first, struct task_struct, pids[(type)].node);
264 if (result && (pid->nr != 1) &&
265 !vx_check(vx_task_xid(result), VX_WATCH|VX_ADMIN|VX_IDENT)) {
266 vxwprintk((type == PIDTYPE_PID) && (current->xid),
267 "pid_task(%d,%d): task %p[#%u,%u] did lookup %p[#%u,%u]",
268 pid->nr, type, current, vx_current_xid(), current->pid,
269 result, vx_task_xid(result), result->pid);
277 * Must be called under rcu_read_lock() or with tasklist_lock read-held.
279 struct task_struct *find_task_by_pid_type(int type, int nr)
281 if (type == PIDTYPE_PID)
282 nr = vx_rmap_pid(nr);
283 else if (type == PIDTYPE_REALPID)
285 return pid_task(find_pid(nr), type);
288 EXPORT_SYMBOL(find_task_by_pid_type);
290 struct task_struct *fastcall get_pid_task(struct pid *pid, enum pid_type type)
292 struct task_struct *result;
294 result = pid_task(pid, type);
296 get_task_struct(result);
301 struct pid *find_get_pid(pid_t nr)
306 pid = get_pid(find_pid(nr));
313 * The pid hash table is scaled according to the amount of memory in the
314 * machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or
317 void __init pidhash_init(void)
320 unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
322 pidhash_shift = max(4, fls(megabytes * 4));
323 pidhash_shift = min(12, pidhash_shift);
324 pidhash_size = 1 << pidhash_shift;
326 printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
327 pidhash_size, pidhash_shift,
328 pidhash_size * sizeof(struct hlist_head));
330 pid_hash = alloc_bootmem(pidhash_size * sizeof(*(pid_hash)));
332 panic("Could not alloc pidhash!\n");
333 for (i = 0; i < pidhash_size; i++)
334 INIT_HLIST_HEAD(&pid_hash[i]);
337 void __init pidmap_init(void)
339 pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
340 /* Reserve PID 0. We never call free_pidmap(0) */
341 set_bit(0, pidmap_array->page);
342 atomic_dec(&pidmap_array->nr_free);
344 pid_cachep = kmem_cache_create("pid", sizeof(struct pid),
345 __alignof__(struct pid),
346 SLAB_PANIC, NULL, NULL);