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/pid_namespace.h>
30 #include <linux/init_task.h>
32 #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
33 static struct hlist_head
*pid_hash
;
34 static int pidhash_shift
;
35 static struct kmem_cache
*pid_cachep
;
36 struct pid init_struct_pid
= INIT_STRUCT_PID
;
38 int pid_max
= PID_MAX_DEFAULT
;
40 #define RESERVED_PIDS 300
42 int pid_max_min
= RESERVED_PIDS
+ 1;
43 int pid_max_max
= PID_MAX_LIMIT
;
45 #define BITS_PER_PAGE (PAGE_SIZE*8)
46 #define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)
48 static inline int mk_pid(struct pid_namespace
*pid_ns
,
49 struct pidmap
*map
, int off
)
51 return (map
- pid_ns
->pidmap
)*BITS_PER_PAGE
+ off
;
54 #define find_next_offset(map, off) \
55 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
58 * PID-map pages start out as NULL, they get allocated upon
59 * first use and are never deallocated. This way a low pid_max
60 * value does not cause lots of bitmaps to be allocated, but
61 * the scheme scales to up to 4 million PIDs, runtime.
63 struct pid_namespace init_pid_ns
= {
65 .refcount
= ATOMIC_INIT(2),
68 [ 0 ... PIDMAP_ENTRIES
-1] = { ATOMIC_INIT(BITS_PER_PAGE
), NULL
}
71 .child_reaper
= &init_task
75 * Note: disable interrupts while the pidmap_lock is held as an
76 * interrupt might come in and do read_lock(&tasklist_lock).
78 * If we don't disable interrupts there is a nasty deadlock between
79 * detach_pid()->free_pid() and another cpu that does
80 * spin_lock(&pidmap_lock) followed by an interrupt routine that does
81 * read_lock(&tasklist_lock);
83 * After we clean up the tasklist_lock and know there are no
84 * irq handlers that take it we can leave the interrupts enabled.
85 * For now it is easier to be safe than to prove it can't happen.
88 static __cacheline_aligned_in_smp
DEFINE_SPINLOCK(pidmap_lock
);
90 static fastcall
void free_pidmap(struct pid_namespace
*pid_ns
, int pid
)
92 struct pidmap
*map
= pid_ns
->pidmap
+ pid
/ BITS_PER_PAGE
;
93 int offset
= pid
& BITS_PER_PAGE_MASK
;
95 clear_bit(offset
, map
->page
);
96 atomic_inc(&map
->nr_free
);
99 static int alloc_pidmap(struct pid_namespace
*pid_ns
)
101 int i
, offset
, max_scan
, pid
, last
= pid_ns
->last_pid
;
107 offset
= pid
& BITS_PER_PAGE_MASK
;
108 map
= &pid_ns
->pidmap
[pid
/BITS_PER_PAGE
];
109 max_scan
= (pid_max
+ BITS_PER_PAGE
- 1)/BITS_PER_PAGE
- !offset
;
110 for (i
= 0; i
<= max_scan
; ++i
) {
111 if (unlikely(!map
->page
)) {
112 void *page
= kzalloc(PAGE_SIZE
, GFP_KERNEL
);
114 * Free the page if someone raced with us
117 spin_lock_irq(&pidmap_lock
);
122 spin_unlock_irq(&pidmap_lock
);
123 if (unlikely(!map
->page
))
126 if (likely(atomic_read(&map
->nr_free
))) {
128 if (!test_and_set_bit(offset
, map
->page
)) {
129 atomic_dec(&map
->nr_free
);
130 pid_ns
->last_pid
= pid
;
133 offset
= find_next_offset(map
, offset
);
134 pid
= mk_pid(pid_ns
, map
, offset
);
136 * find_next_offset() found a bit, the pid from it
137 * is in-bounds, and if we fell back to the last
138 * bitmap block and the final block was the same
139 * as the starting point, pid is before last_pid.
141 } while (offset
< BITS_PER_PAGE
&& pid
< pid_max
&&
142 (i
!= max_scan
|| pid
< last
||
143 !((last
+1) & BITS_PER_PAGE_MASK
)));
145 if (map
< &pid_ns
->pidmap
[(pid_max
-1)/BITS_PER_PAGE
]) {
149 map
= &pid_ns
->pidmap
[0];
150 offset
= RESERVED_PIDS
;
151 if (unlikely(last
== offset
))
154 pid
= mk_pid(pid_ns
, map
, offset
);
159 static int next_pidmap(struct pid_namespace
*pid_ns
, int last
)
162 struct pidmap
*map
, *end
;
164 offset
= (last
+ 1) & BITS_PER_PAGE_MASK
;
165 map
= &pid_ns
->pidmap
[(last
+ 1)/BITS_PER_PAGE
];
166 end
= &pid_ns
->pidmap
[PIDMAP_ENTRIES
];
167 for (; map
< end
; map
++, offset
= 0) {
168 if (unlikely(!map
->page
))
170 offset
= find_next_bit((map
)->page
, BITS_PER_PAGE
, offset
);
171 if (offset
< BITS_PER_PAGE
)
172 return mk_pid(pid_ns
, map
, offset
);
177 fastcall
void put_pid(struct pid
*pid
)
181 if ((atomic_read(&pid
->count
) == 1) ||
182 atomic_dec_and_test(&pid
->count
))
183 kmem_cache_free(pid_cachep
, pid
);
185 EXPORT_SYMBOL_GPL(put_pid
);
187 static void delayed_put_pid(struct rcu_head
*rhp
)
189 struct pid
*pid
= container_of(rhp
, struct pid
, rcu
);
193 fastcall
void free_pid(struct pid
*pid
)
195 /* We can be called with write_lock_irq(&tasklist_lock) held */
198 spin_lock_irqsave(&pidmap_lock
, flags
);
199 hlist_del_rcu(&pid
->pid_chain
);
200 spin_unlock_irqrestore(&pidmap_lock
, flags
);
202 free_pidmap(&init_pid_ns
, pid
->nr
);
203 call_rcu(&pid
->rcu
, delayed_put_pid
);
206 struct pid
*alloc_pid(void)
212 pid
= kmem_cache_alloc(pid_cachep
, GFP_KERNEL
);
216 nr
= alloc_pidmap(current
->nsproxy
->pid_ns
);
220 atomic_set(&pid
->count
, 1);
222 for (type
= 0; type
< PIDTYPE_MAX
; ++type
)
223 INIT_HLIST_HEAD(&pid
->tasks
[type
]);
225 spin_lock_irq(&pidmap_lock
);
226 hlist_add_head_rcu(&pid
->pid_chain
, &pid_hash
[pid_hashfn(pid
->nr
)]);
227 spin_unlock_irq(&pidmap_lock
);
233 kmem_cache_free(pid_cachep
, pid
);
238 struct pid
* fastcall
find_pid(int nr
)
240 struct hlist_node
*elem
;
243 hlist_for_each_entry_rcu(pid
, elem
,
244 &pid_hash
[pid_hashfn(nr
)], pid_chain
) {
250 EXPORT_SYMBOL_GPL(find_pid
);
253 * attach_pid() must be called with the tasklist_lock write-held.
255 int fastcall
attach_pid(struct task_struct
*task
, enum pid_type type
,
258 struct pid_link
*link
;
260 link
= &task
->pids
[type
];
262 hlist_add_head_rcu(&link
->node
, &pid
->tasks
[type
]);
267 void fastcall
detach_pid(struct task_struct
*task
, enum pid_type type
)
269 struct pid_link
*link
;
273 link
= &task
->pids
[type
];
276 hlist_del_rcu(&link
->node
);
279 for (tmp
= PIDTYPE_MAX
; --tmp
>= 0; )
280 if (!hlist_empty(&pid
->tasks
[tmp
]))
286 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
287 void fastcall
transfer_pid(struct task_struct
*old
, struct task_struct
*new,
290 new->pids
[type
].pid
= old
->pids
[type
].pid
;
291 hlist_replace_rcu(&old
->pids
[type
].node
, &new->pids
[type
].node
);
292 old
->pids
[type
].pid
= NULL
;
295 struct task_struct
* fastcall
pid_task(struct pid
*pid
, enum pid_type type
)
297 struct task_struct
*result
= NULL
;
299 struct hlist_node
*first
;
300 first
= rcu_dereference(pid
->tasks
[type
].first
);
302 result
= hlist_entry(first
, struct task_struct
, pids
[(type
)].node
);
308 * Must be called under rcu_read_lock() or with tasklist_lock read-held.
310 struct task_struct
*find_task_by_pid_type(int type
, int nr
)
312 return pid_task(find_pid(nr
), type
);
315 EXPORT_SYMBOL(find_task_by_pid_type
);
317 struct pid
*get_task_pid(struct task_struct
*task
, enum pid_type type
)
321 pid
= get_pid(task
->pids
[type
].pid
);
326 struct task_struct
*fastcall
get_pid_task(struct pid
*pid
, enum pid_type type
)
328 struct task_struct
*result
;
330 result
= pid_task(pid
, type
);
332 get_task_struct(result
);
337 struct pid
*find_get_pid(pid_t nr
)
342 pid
= get_pid(find_pid(nr
));
349 * Used by proc to find the first pid that is greater then or equal to nr.
351 * If there is a pid at nr this function is exactly the same as find_pid.
353 struct pid
*find_ge_pid(int nr
)
361 nr
= next_pidmap(current
->nsproxy
->pid_ns
, nr
);
366 EXPORT_SYMBOL_GPL(find_get_pid
);
368 struct pid_namespace
*copy_pid_ns(unsigned long flags
, struct pid_namespace
*old_ns
)
375 void free_pid_ns(struct kref
*kref
)
377 struct pid_namespace
*ns
;
379 ns
= container_of(kref
, struct pid_namespace
, kref
);
384 * The pid hash table is scaled according to the amount of memory in the
385 * machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or
388 void __init
pidhash_init(void)
391 unsigned long megabytes
= nr_kernel_pages
>> (20 - PAGE_SHIFT
);
393 pidhash_shift
= max(4, fls(megabytes
* 4));
394 pidhash_shift
= min(12, pidhash_shift
);
395 pidhash_size
= 1 << pidhash_shift
;
397 printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
398 pidhash_size
, pidhash_shift
,
399 pidhash_size
* sizeof(struct hlist_head
));
401 pid_hash
= alloc_bootmem(pidhash_size
* sizeof(*(pid_hash
)));
403 panic("Could not alloc pidhash!\n");
404 for (i
= 0; i
< pidhash_size
; i
++)
405 INIT_HLIST_HEAD(&pid_hash
[i
]);
408 void __init
pidmap_init(void)
410 init_pid_ns
.pidmap
[0].page
= kzalloc(PAGE_SIZE
, GFP_KERNEL
);
411 /* Reserve PID 0. We never call free_pidmap(0) */
412 set_bit(0, init_pid_ns
.pidmap
[0].page
);
413 atomic_dec(&init_pid_ns
.pidmap
[0].nr_free
);
415 pid_cachep
= KMEM_CACHE(pid
, SLAB_PANIC
);