Root/kernel/cgroup_freezer.c

1/*
2 * cgroup_freezer.c - control group freezer subsystem
3 *
4 * Copyright IBM Corporation, 2007
5 *
6 * Author : Cedric Le Goater <clg@fr.ibm.com>
7 *
8 * This program is free software; you can redistribute it and/or modify it
9 * under the terms of version 2.1 of the GNU Lesser General Public License
10 * as published by the Free Software Foundation.
11 *
12 * This program is distributed in the hope that it would be useful, but
13 * WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
15 */
16
17#include <linux/export.h>
18#include <linux/slab.h>
19#include <linux/cgroup.h>
20#include <linux/fs.h>
21#include <linux/uaccess.h>
22#include <linux/freezer.h>
23#include <linux/seq_file.h>
24
25enum freezer_state {
26    CGROUP_THAWED = 0,
27    CGROUP_FREEZING,
28    CGROUP_FROZEN,
29};
30
31struct freezer {
32    struct cgroup_subsys_state css;
33    enum freezer_state state;
34    spinlock_t lock; /* protects _writes_ to state */
35};
36
37static inline struct freezer *cgroup_freezer(
38        struct cgroup *cgroup)
39{
40    return container_of(
41        cgroup_subsys_state(cgroup, freezer_subsys_id),
42        struct freezer, css);
43}
44
45static inline struct freezer *task_freezer(struct task_struct *task)
46{
47    return container_of(task_subsys_state(task, freezer_subsys_id),
48                struct freezer, css);
49}
50
51bool cgroup_freezing(struct task_struct *task)
52{
53    enum freezer_state state;
54    bool ret;
55
56    rcu_read_lock();
57    state = task_freezer(task)->state;
58    ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
59    rcu_read_unlock();
60
61    return ret;
62}
63
64/*
65 * cgroups_write_string() limits the size of freezer state strings to
66 * CGROUP_LOCAL_BUFFER_SIZE
67 */
68static const char *freezer_state_strs[] = {
69    "THAWED",
70    "FREEZING",
71    "FROZEN",
72};
73
74/*
75 * State diagram
76 * Transitions are caused by userspace writes to the freezer.state file.
77 * The values in parenthesis are state labels. The rest are edge labels.
78 *
79 * (THAWED) --FROZEN--> (FREEZING) --FROZEN--> (FROZEN)
80 * ^ ^ | |
81 * | \_______THAWED_______/ |
82 * \__________________________THAWED____________/
83 */
84
85struct cgroup_subsys freezer_subsys;
86
87/* Locks taken and their ordering
88 * ------------------------------
89 * cgroup_mutex (AKA cgroup_lock)
90 * freezer->lock
91 * css_set_lock
92 * task->alloc_lock (AKA task_lock)
93 * task->sighand->siglock
94 *
95 * cgroup code forces css_set_lock to be taken before task->alloc_lock
96 *
97 * freezer_create(), freezer_destroy():
98 * cgroup_mutex [ by cgroup core ]
99 *
100 * freezer_can_attach():
101 * cgroup_mutex (held by caller of can_attach)
102 *
103 * freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
104 * freezer->lock
105 * sighand->siglock (if the cgroup is freezing)
106 *
107 * freezer_read():
108 * cgroup_mutex
109 * freezer->lock
110 * write_lock css_set_lock (cgroup iterator start)
111 * task->alloc_lock
112 * read_lock css_set_lock (cgroup iterator start)
113 *
114 * freezer_write() (freeze):
115 * cgroup_mutex
116 * freezer->lock
117 * write_lock css_set_lock (cgroup iterator start)
118 * task->alloc_lock
119 * read_lock css_set_lock (cgroup iterator start)
120 * sighand->siglock (fake signal delivery inside freeze_task())
121 *
122 * freezer_write() (unfreeze):
123 * cgroup_mutex
124 * freezer->lock
125 * write_lock css_set_lock (cgroup iterator start)
126 * task->alloc_lock
127 * read_lock css_set_lock (cgroup iterator start)
128 * task->alloc_lock (inside __thaw_task(), prevents race with refrigerator())
129 * sighand->siglock
130 */
131static struct cgroup_subsys_state *freezer_create(struct cgroup *cgroup)
132{
133    struct freezer *freezer;
134
135    freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
136    if (!freezer)
137        return ERR_PTR(-ENOMEM);
138
139    spin_lock_init(&freezer->lock);
140    freezer->state = CGROUP_THAWED;
141    return &freezer->css;
142}
143
144static void freezer_destroy(struct cgroup *cgroup)
145{
146    struct freezer *freezer = cgroup_freezer(cgroup);
147
148    if (freezer->state != CGROUP_THAWED)
149        atomic_dec(&system_freezing_cnt);
150    kfree(freezer);
151}
152
153/* task is frozen or will freeze immediately when next it gets woken */
154static bool is_task_frozen_enough(struct task_struct *task)
155{
156    return frozen(task) ||
157        (task_is_stopped_or_traced(task) && freezing(task));
158}
159
160/*
161 * The call to cgroup_lock() in the freezer.state write method prevents
162 * a write to that file racing against an attach, and hence the
163 * can_attach() result will remain valid until the attach completes.
164 */
165static int freezer_can_attach(struct cgroup *new_cgroup,
166                  struct cgroup_taskset *tset)
167{
168    struct freezer *freezer;
169    struct task_struct *task;
170
171    /*
172     * Anything frozen can't move or be moved to/from.
173     */
174    cgroup_taskset_for_each(task, new_cgroup, tset)
175        if (cgroup_freezing(task))
176            return -EBUSY;
177
178    freezer = cgroup_freezer(new_cgroup);
179    if (freezer->state != CGROUP_THAWED)
180        return -EBUSY;
181
182    return 0;
183}
184
185static void freezer_fork(struct task_struct *task)
186{
187    struct freezer *freezer;
188
189    /*
190     * No lock is needed, since the task isn't on tasklist yet,
191     * so it can't be moved to another cgroup, which means the
192     * freezer won't be removed and will be valid during this
193     * function call. Nevertheless, apply RCU read-side critical
194     * section to suppress RCU lockdep false positives.
195     */
196    rcu_read_lock();
197    freezer = task_freezer(task);
198    rcu_read_unlock();
199
200    /*
201     * The root cgroup is non-freezable, so we can skip the
202     * following check.
203     */
204    if (!freezer->css.cgroup->parent)
205        return;
206
207    spin_lock_irq(&freezer->lock);
208    BUG_ON(freezer->state == CGROUP_FROZEN);
209
210    /* Locking avoids race with FREEZING -> THAWED transitions. */
211    if (freezer->state == CGROUP_FREEZING)
212        freeze_task(task);
213    spin_unlock_irq(&freezer->lock);
214}
215
216/*
217 * caller must hold freezer->lock
218 */
219static void update_if_frozen(struct cgroup *cgroup,
220                 struct freezer *freezer)
221{
222    struct cgroup_iter it;
223    struct task_struct *task;
224    unsigned int nfrozen = 0, ntotal = 0;
225    enum freezer_state old_state = freezer->state;
226
227    cgroup_iter_start(cgroup, &it);
228    while ((task = cgroup_iter_next(cgroup, &it))) {
229        ntotal++;
230        if (freezing(task) && is_task_frozen_enough(task))
231            nfrozen++;
232    }
233
234    if (old_state == CGROUP_THAWED) {
235        BUG_ON(nfrozen > 0);
236    } else if (old_state == CGROUP_FREEZING) {
237        if (nfrozen == ntotal)
238            freezer->state = CGROUP_FROZEN;
239    } else { /* old_state == CGROUP_FROZEN */
240        BUG_ON(nfrozen != ntotal);
241    }
242
243    cgroup_iter_end(cgroup, &it);
244}
245
246static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
247            struct seq_file *m)
248{
249    struct freezer *freezer;
250    enum freezer_state state;
251
252    if (!cgroup_lock_live_group(cgroup))
253        return -ENODEV;
254
255    freezer = cgroup_freezer(cgroup);
256    spin_lock_irq(&freezer->lock);
257    state = freezer->state;
258    if (state == CGROUP_FREEZING) {
259        /* We change from FREEZING to FROZEN lazily if the cgroup was
260         * only partially frozen when we exitted write. */
261        update_if_frozen(cgroup, freezer);
262        state = freezer->state;
263    }
264    spin_unlock_irq(&freezer->lock);
265    cgroup_unlock();
266
267    seq_puts(m, freezer_state_strs[state]);
268    seq_putc(m, '\n');
269    return 0;
270}
271
272static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
273{
274    struct cgroup_iter it;
275    struct task_struct *task;
276    unsigned int num_cant_freeze_now = 0;
277
278    cgroup_iter_start(cgroup, &it);
279    while ((task = cgroup_iter_next(cgroup, &it))) {
280        if (!freeze_task(task))
281            continue;
282        if (is_task_frozen_enough(task))
283            continue;
284        if (!freezing(task) && !freezer_should_skip(task))
285            num_cant_freeze_now++;
286    }
287    cgroup_iter_end(cgroup, &it);
288
289    return num_cant_freeze_now ? -EBUSY : 0;
290}
291
292static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
293{
294    struct cgroup_iter it;
295    struct task_struct *task;
296
297    cgroup_iter_start(cgroup, &it);
298    while ((task = cgroup_iter_next(cgroup, &it)))
299        __thaw_task(task);
300    cgroup_iter_end(cgroup, &it);
301}
302
303static int freezer_change_state(struct cgroup *cgroup,
304                enum freezer_state goal_state)
305{
306    struct freezer *freezer;
307    int retval = 0;
308
309    freezer = cgroup_freezer(cgroup);
310
311    spin_lock_irq(&freezer->lock);
312
313    update_if_frozen(cgroup, freezer);
314
315    switch (goal_state) {
316    case CGROUP_THAWED:
317        if (freezer->state != CGROUP_THAWED)
318            atomic_dec(&system_freezing_cnt);
319        freezer->state = CGROUP_THAWED;
320        unfreeze_cgroup(cgroup, freezer);
321        break;
322    case CGROUP_FROZEN:
323        if (freezer->state == CGROUP_THAWED)
324            atomic_inc(&system_freezing_cnt);
325        freezer->state = CGROUP_FREEZING;
326        retval = try_to_freeze_cgroup(cgroup, freezer);
327        break;
328    default:
329        BUG();
330    }
331
332    spin_unlock_irq(&freezer->lock);
333
334    return retval;
335}
336
337static int freezer_write(struct cgroup *cgroup,
338             struct cftype *cft,
339             const char *buffer)
340{
341    int retval;
342    enum freezer_state goal_state;
343
344    if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
345        goal_state = CGROUP_THAWED;
346    else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
347        goal_state = CGROUP_FROZEN;
348    else
349        return -EINVAL;
350
351    if (!cgroup_lock_live_group(cgroup))
352        return -ENODEV;
353    retval = freezer_change_state(cgroup, goal_state);
354    cgroup_unlock();
355    return retval;
356}
357
358static struct cftype files[] = {
359    {
360        .name = "state",
361        .flags = CFTYPE_NOT_ON_ROOT,
362        .read_seq_string = freezer_read,
363        .write_string = freezer_write,
364    },
365    { } /* terminate */
366};
367
368struct cgroup_subsys freezer_subsys = {
369    .name = "freezer",
370    .create = freezer_create,
371    .destroy = freezer_destroy,
372    .subsys_id = freezer_subsys_id,
373    .can_attach = freezer_can_attach,
374    .fork = freezer_fork,
375    .base_cftypes = files,
376};
377

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