projects/linux·blob v2.6.16-rc6 · refs

292 lines · 7.6 KiB

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)
31static struct hlist_head *pid_hash[PIDTYPE_MAX];
32static int pidhash_shift;
33
34int pid_max = PID_MAX_DEFAULT;
35int last_pid;
36
37#define RESERVED_PIDS 300
38
39int pid_max_min = RESERVED_PIDS + 1;
40int pid_max_max = PID_MAX_LIMIT;
41
42#define PIDMAP_ENTRIES ((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8)
43#define BITS_PER_PAGE (PAGE_SIZE*8)
44#define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)
45#define mk_pid(map, off) (((map) - pidmap_array)*BITS_PER_PAGE + (off))
46#define find_next_offset(map, off) \
47 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
48
49/*
50 * PID-map pages start out as NULL, they get allocated upon
51 * first use and are never deallocated. This way a low pid_max
52 * value does not cause lots of bitmaps to be allocated, but
53 * the scheme scales to up to 4 million PIDs, runtime.
54 */
55typedef struct pidmap {
56 atomic_t nr_free;
57 void *page;
58} pidmap_t;
59
60static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
61 { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };
62
63static __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
64
65fastcall void free_pidmap(int pid)
66{
67 pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
68 int offset = pid & BITS_PER_PAGE_MASK;
69
70 clear_bit(offset, map->page);
71 atomic_inc(&map->nr_free);
72}
73
74int alloc_pidmap(void)
75{
76 int i, offset, max_scan, pid, last = last_pid;
77 pidmap_t *map;
78
79 pid = last + 1;
80 if (pid >= pid_max)
81 pid = RESERVED_PIDS;
82 offset = pid & BITS_PER_PAGE_MASK;
83 map = &pidmap_array[pid/BITS_PER_PAGE];
84 max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
85 for (i = 0; i <= max_scan; ++i) {
86 if (unlikely(!map->page)) {
87 unsigned long page = get_zeroed_page(GFP_KERNEL);
88 /*
89 * Free the page if someone raced with us
90 * installing it:
91 */
92 spin_lock(&pidmap_lock);
93 if (map->page)
94 free_page(page);
95 else
96 map->page = (void *)page;
97 spin_unlock(&pidmap_lock);
98 if (unlikely(!map->page))
99 break;
100 }
101 if (likely(atomic_read(&map->nr_free))) {
102 do {
103 if (!test_and_set_bit(offset, map->page)) {
104 atomic_dec(&map->nr_free);
105 last_pid = pid;
106 return pid;
107 }
108 offset = find_next_offset(map, offset);
109 pid = mk_pid(map, offset);
110 /*
111 * find_next_offset() found a bit, the pid from it
112 * is in-bounds, and if we fell back to the last
113 * bitmap block and the final block was the same
114 * as the starting point, pid is before last_pid.
115 */
116 } while (offset < BITS_PER_PAGE && pid < pid_max &&
117 (i != max_scan || pid < last ||
118 !((last+1) & BITS_PER_PAGE_MASK)));
119 }
120 if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
121 ++map;
122 offset = 0;
123 } else {
124 map = &pidmap_array[0];
125 offset = RESERVED_PIDS;
126 if (unlikely(last == offset))
127 break;
128 }
129 pid = mk_pid(map, offset);
130 }
131 return -1;
132}
133
134struct pid * fastcall find_pid(enum pid_type type, int nr)
135{
136 struct hlist_node *elem;
137 struct pid *pid;
138
139 hlist_for_each_entry_rcu(pid, elem,
140 &pid_hash[type][pid_hashfn(nr)], pid_chain) {
141 if (pid->nr == nr)
142 return pid;
143 }
144 return NULL;
145}
146
147int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
148{
149 struct pid *pid, *task_pid;
150
151 task_pid = &task->pids[type];
152 pid = find_pid(type, nr);
153 task_pid->nr = nr;
154 if (pid == NULL) {
155 INIT_LIST_HEAD(&task_pid->pid_list);
156 hlist_add_head_rcu(&task_pid->pid_chain,
157 &pid_hash[type][pid_hashfn(nr)]);
158 } else {
159 INIT_HLIST_NODE(&task_pid->pid_chain);
160 list_add_tail_rcu(&task_pid->pid_list, &pid->pid_list);
161 }
162
163 return 0;
164}
165
166static fastcall int __detach_pid(task_t *task, enum pid_type type)
167{
168 struct pid *pid, *pid_next;
169 int nr = 0;
170
171 pid = &task->pids[type];
172 if (!hlist_unhashed(&pid->pid_chain)) {
173
174 if (list_empty(&pid->pid_list)) {
175 nr = pid->nr;
176 hlist_del_rcu(&pid->pid_chain);
177 } else {
178 pid_next = list_entry(pid->pid_list.next,
179 struct pid, pid_list);
180 /* insert next pid from pid_list to hash */
181 hlist_replace_rcu(&pid->pid_chain,
182 &pid_next->pid_chain);
183 }
184 }
185
186 list_del_rcu(&pid->pid_list);
187 pid->nr = 0;
188
189 return nr;
190}
191
192void fastcall detach_pid(task_t *task, enum pid_type type)
193{
194 int tmp, nr;
195
196 nr = __detach_pid(task, type);
197 if (!nr)
198 return;
199
200 for (tmp = PIDTYPE_MAX; --tmp >= 0; )
201 if (tmp != type && find_pid(tmp, nr))
202 return;
203
204 free_pidmap(nr);
205}
206
207task_t *find_task_by_pid_type(int type, int nr)
208{
209 struct pid *pid;
210
211 pid = find_pid(type, nr);
212 if (!pid)
213 return NULL;
214
215 return pid_task(&pid->pid_list, type);
216}
217
218EXPORT_SYMBOL(find_task_by_pid_type);
219
220/*
221 * This function switches the PIDs if a non-leader thread calls
222 * sys_execve() - this must be done without releasing the PID.
223 * (which a detach_pid() would eventually do.)
224 */
225void switch_exec_pids(task_t *leader, task_t *thread)
226{
227 __detach_pid(leader, PIDTYPE_PID);
228 __detach_pid(leader, PIDTYPE_TGID);
229 __detach_pid(leader, PIDTYPE_PGID);
230 __detach_pid(leader, PIDTYPE_SID);
231
232 __detach_pid(thread, PIDTYPE_PID);
233 __detach_pid(thread, PIDTYPE_TGID);
234
235 leader->pid = leader->tgid = thread->pid;
236 thread->pid = thread->tgid;
237
238 attach_pid(thread, PIDTYPE_PID, thread->pid);
239 attach_pid(thread, PIDTYPE_TGID, thread->tgid);
240 attach_pid(thread, PIDTYPE_PGID, thread->signal->pgrp);
241 attach_pid(thread, PIDTYPE_SID, thread->signal->session);
242 list_add_tail(&thread->tasks, &init_task.tasks);
243
244 attach_pid(leader, PIDTYPE_PID, leader->pid);
245 attach_pid(leader, PIDTYPE_TGID, leader->tgid);
246 attach_pid(leader, PIDTYPE_PGID, leader->signal->pgrp);
247 attach_pid(leader, PIDTYPE_SID, leader->signal->session);
248}
249
250/*
251 * The pid hash table is scaled according to the amount of memory in the
252 * machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or
253 * more.
254 */
255void __init pidhash_init(void)
256{
257 int i, j, pidhash_size;
258 unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
259
260 pidhash_shift = max(4, fls(megabytes * 4));
261 pidhash_shift = min(12, pidhash_shift);
262 pidhash_size = 1 << pidhash_shift;
263
264 printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
265 pidhash_size, pidhash_shift,
266 PIDTYPE_MAX * pidhash_size * sizeof(struct hlist_head));
267
268 for (i = 0; i < PIDTYPE_MAX; i++) {
269 pid_hash[i] = alloc_bootmem(pidhash_size *
270 sizeof(*(pid_hash[i])));
271 if (!pid_hash[i])
272 panic("Could not alloc pidhash!\n");
273 for (j = 0; j < pidhash_size; j++)
274 INIT_HLIST_HEAD(&pid_hash[i][j]);
275 }
276}
277
278void __init pidmap_init(void)
279{
280 int i;
281
282 pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
283 set_bit(0, pidmap_array->page);
284 atomic_dec(&pidmap_array->nr_free);
285
286 /*
287 * Allocate PID 0, and hash it via all PID types:
288 */
289
290 for (i = 0; i < PIDTYPE_MAX; i++)
291 attach_pid(current, i, 0);
292}