summaryrefslogtreecommitdiffstats
path: root/kernel/pid.c
blob: a9f2dfd006d2d7ddee6f6413cbe82d793a6fc3e7 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
/*
 * Generic pidhash and scalable, time-bounded PID allocator
 *
 * (C) 2002-2003 William Irwin, IBM
 * (C) 2004 William Irwin, Oracle
 * (C) 2002-2004 Ingo Molnar, Red Hat
 *
 * pid-structures are backing objects for tasks sharing a given ID to chain
 * against. There is very little to them aside from hashing them and
 * parking tasks using given ID's on a list.
 *
 * The hash is always changed with the tasklist_lock write-acquired,
 * and the hash is only accessed with the tasklist_lock at least
 * read-acquired, so there's no additional SMP locking needed here.
 *
 * We have a list of bitmap pages, which bitmaps represent the PID space.
 * Allocating and freeing PIDs is completely lockless. The worst-case
 * allocation scenario when all but one out of 1 million PIDs possible are
 * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
 * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/bootmem.h>
#include <linux/hash.h>

#define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
static struct hlist_head *pid_hash[PIDTYPE_MAX];
static int pidhash_shift;

int pid_max = PID_MAX_DEFAULT;
int last_pid;

#define RESERVED_PIDS		300

int pid_max_min = RESERVED_PIDS + 1;
int pid_max_max = PID_MAX_LIMIT;

#define PIDMAP_ENTRIES		((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8)
#define BITS_PER_PAGE		(PAGE_SIZE*8)
#define BITS_PER_PAGE_MASK	(BITS_PER_PAGE-1)
#define mk_pid(map, off)	(((map) - pidmap_array)*BITS_PER_PAGE + (off))
#define find_next_offset(map, off)					\
		find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

/*
 * PID-map pages start out as NULL, they get allocated upon
 * first use and are never deallocated. This way a low pid_max
 * value does not cause lots of bitmaps to be allocated, but
 * the scheme scales to up to 4 million PIDs, runtime.
 */
typedef struct pidmap {
	atomic_t nr_free;
	void *page;
} pidmap_t;

static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
	 { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };

static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

fastcall void free_pidmap(int pid)
{
	pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
	int offset = pid & BITS_PER_PAGE_MASK;

	clear_bit(offset, map->page);
	atomic_inc(&map->nr_free);
}

int alloc_pidmap(void)
{
	int i, offset, max_scan, pid, last = last_pid;
	pidmap_t *map;

	pid = last + 1;
	if (pid >= pid_max)
		pid = RESERVED_PIDS;
	offset = pid & BITS_PER_PAGE_MASK;
	map = &pidmap_array[pid/BITS_PER_PAGE];
	max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
	for (i = 0; i <= max_scan; ++i) {
		if (unlikely(!map->page)) {
			unsigned long page = get_zeroed_page(GFP_KERNEL);
			/*
			 * Free the page if someone raced with us
			 * installing it:
			 */
			spin_lock(&pidmap_lock);
			if (map->page)
				free_page(page);
			else
				map->page = (void *)page;
			spin_unlock(&pidmap_lock);
			if (unlikely(!map->page))
				break;
		}
		if (likely(atomic_read(&map->nr_free))) {
			do {
				if (!test_and_set_bit(offset, map->page)) {
					atomic_dec(&map->nr_free);
					last_pid = pid;
					return pid;
				}
				offset = find_next_offset(map, offset);
				pid = mk_pid(map, offset);
			/*
			 * find_next_offset() found a bit, the pid from it
			 * is in-bounds, and if we fell back to the last
			 * bitmap block and the final block was the same
			 * as the starting point, pid is before last_pid.
			 */
			} while (offset < BITS_PER_PAGE && pid < pid_max &&
					(i != max_scan || pid < last ||
					    !((last+1) & BITS_PER_PAGE_MASK)));
		}
		if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
			++map;
			offset = 0;
		} else {
			map = &pidmap_array[0];
			offset = RESERVED_PIDS;
			if (unlikely(last == offset))
				break;
		}
		pid = mk_pid(map, offset);
	}
	return -1;
}

struct pid * fastcall find_pid(enum pid_type type, int nr)
{
	struct hlist_node *elem;
	struct pid *pid;

	hlist_for_each_entry_rcu(pid, elem,
			&pid_hash[type][pid_hashfn(nr)], pid_chain) {
		if (pid->nr == nr)
			return pid;
	}
	return NULL;
}

int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
{
	struct pid *pid, *task_pid;

	task_pid = &task->pids[type];
	pid = find_pid(type, nr);
	task_pid->nr = nr;
	if (pid == NULL) {
		INIT_LIST_HEAD(&task_pid->pid_list);
		hlist_add_head_rcu(&task_pid->pid_chain,
				   &pid_hash[type][pid_hashfn(nr)]);
	} else {
		INIT_HLIST_NODE(&task_pid->pid_chain);
		list_add_tail_rcu(&task_pid->pid_list, &pid->pid_list);
	}

	return 0;
}

static fastcall int __detach_pid(task_t *task, enum pid_type type)
{
	struct pid *pid, *pid_next;
	int nr = 0;

	pid = &task->pids[type];
	if (!hlist_unhashed(&pid->pid_chain)) {

		if (list_empty(&pid->pid_list)) {
			nr = pid->nr;
			hlist_del_rcu(&pid->pid_chain);
		} else {
			pid_next = list_entry(pid->pid_list.next,
						struct pid, pid_list);
			/* insert next pid from pid_list to hash */
			hlist_replace_rcu(&pid->pid_chain,
					  &pid_next->pid_chain);
		}
	}

	list_del_rcu(&pid->pid_list);
	pid->nr = 0;

	return nr;
}

void fastcall detach_pid(task_t *task, enum pid_type type)
{
	int tmp, nr;

	nr = __detach_pid(task, type);
	if (!nr)
		return;

	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
		if (tmp != type && find_pid(tmp, nr))
			return;

	free_pidmap(nr);
}

task_t *find_task_by_pid_type(int type, int nr)
{
	struct pid *pid;

	pid = find_pid(type, nr);
	if (!pid)
		return NULL;

	return pid_task(&pid->pid_list, type);
}

EXPORT_SYMBOL(find_task_by_pid_type);

/*
 * The pid hash table is scaled according to the amount of memory in the
 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 * more.
 */
void __init pidhash_init(void)
{
	int i, j, pidhash_size;
	unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);

	pidhash_shift = max(4, fls(megabytes * 4));
	pidhash_shift = min(12, pidhash_shift);
	pidhash_size = 1 << pidhash_shift;

	printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
		pidhash_size, pidhash_shift,
		PIDTYPE_MAX * pidhash_size * sizeof(struct hlist_head));

	for (i = 0; i < PIDTYPE_MAX; i++) {
		pid_hash[i] = alloc_bootmem(pidhash_size *
					sizeof(*(pid_hash[i])));
		if (!pid_hash[i])
			panic("Could not alloc pidhash!\n");
		for (j = 0; j < pidhash_size; j++)
			INIT_HLIST_HEAD(&pid_hash[i][j]);
	}
}

void __init pidmap_init(void)
{
	pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
	/* Reserve PID 0. We never call free_pidmap(0) */
	set_bit(0, pidmap_array->page);
	atomic_dec(&pidmap_array->nr_free);
}