freezer_cg: disable writing freezer.state of root cgroup
[linux-2.6.git] / 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/module.h>
18 #include <linux/cgroup.h>
19 #include <linux/fs.h>
20 #include <linux/uaccess.h>
21 #include <linux/freezer.h>
22 #include <linux/seq_file.h>
23
24 enum freezer_state {
25         CGROUP_THAWED = 0,
26         CGROUP_FREEZING,
27         CGROUP_FROZEN,
28 };
29
30 struct freezer {
31         struct cgroup_subsys_state css;
32         enum freezer_state state;
33         spinlock_t lock; /* protects _writes_ to state */
34 };
35
36 static inline struct freezer *cgroup_freezer(
37                 struct cgroup *cgroup)
38 {
39         return container_of(
40                 cgroup_subsys_state(cgroup, freezer_subsys_id),
41                 struct freezer, css);
42 }
43
44 static inline struct freezer *task_freezer(struct task_struct *task)
45 {
46         return container_of(task_subsys_state(task, freezer_subsys_id),
47                             struct freezer, css);
48 }
49
50 int cgroup_frozen(struct task_struct *task)
51 {
52         struct freezer *freezer;
53         enum freezer_state state;
54
55         task_lock(task);
56         freezer = task_freezer(task);
57         state = freezer->state;
58         task_unlock(task);
59
60         return state == CGROUP_FROZEN;
61 }
62
63 /*
64  * cgroups_write_string() limits the size of freezer state strings to
65  * CGROUP_LOCAL_BUFFER_SIZE
66  */
67 static const char *freezer_state_strs[] = {
68         "THAWED",
69         "FREEZING",
70         "FROZEN",
71 };
72
73 /*
74  * State diagram
75  * Transitions are caused by userspace writes to the freezer.state file.
76  * The values in parenthesis are state labels. The rest are edge labels.
77  *
78  * (THAWED) --FROZEN--> (FREEZING) --FROZEN--> (FROZEN)
79  *    ^ ^                    |                     |
80  *    | \_______THAWED_______/                     |
81  *    \__________________________THAWED____________/
82  */
83
84 struct cgroup_subsys freezer_subsys;
85
86 /* Locks taken and their ordering
87  * ------------------------------
88  * css_set_lock
89  * cgroup_mutex (AKA cgroup_lock)
90  * task->alloc_lock (AKA task_lock)
91  * freezer->lock
92  * task->sighand->siglock
93  *
94  * cgroup code forces css_set_lock to be taken before task->alloc_lock
95  *
96  * freezer_create(), freezer_destroy():
97  * cgroup_mutex [ by cgroup core ]
98  *
99  * can_attach():
100  * cgroup_mutex
101  *
102  * cgroup_frozen():
103  * task->alloc_lock (to get task's cgroup)
104  *
105  * freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
106  * task->alloc_lock (to get task's cgroup)
107  * freezer->lock
108  *  sighand->siglock (if the cgroup is freezing)
109  *
110  * freezer_read():
111  * cgroup_mutex
112  *  freezer->lock
113  *   read_lock css_set_lock (cgroup iterator start)
114  *
115  * freezer_write() (freeze):
116  * cgroup_mutex
117  *  freezer->lock
118  *   read_lock css_set_lock (cgroup iterator start)
119  *    sighand->siglock
120  *
121  * freezer_write() (unfreeze):
122  * cgroup_mutex
123  *  freezer->lock
124  *   read_lock css_set_lock (cgroup iterator start)
125  *    task->alloc_lock (to prevent races with freeze_task())
126  *     sighand->siglock
127  */
128 static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
129                                                   struct cgroup *cgroup)
130 {
131         struct freezer *freezer;
132
133         freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
134         if (!freezer)
135                 return ERR_PTR(-ENOMEM);
136
137         spin_lock_init(&freezer->lock);
138         freezer->state = CGROUP_THAWED;
139         return &freezer->css;
140 }
141
142 static void freezer_destroy(struct cgroup_subsys *ss,
143                             struct cgroup *cgroup)
144 {
145         kfree(cgroup_freezer(cgroup));
146 }
147
148 /* Task is frozen or will freeze immediately when next it gets woken */
149 static bool is_task_frozen_enough(struct task_struct *task)
150 {
151         return frozen(task) ||
152                 (task_is_stopped_or_traced(task) && freezing(task));
153 }
154
155 /*
156  * The call to cgroup_lock() in the freezer.state write method prevents
157  * a write to that file racing against an attach, and hence the
158  * can_attach() result will remain valid until the attach completes.
159  */
160 static int freezer_can_attach(struct cgroup_subsys *ss,
161                               struct cgroup *new_cgroup,
162                               struct task_struct *task)
163 {
164         struct freezer *freezer;
165
166         /*
167          * Anything frozen can't move or be moved to/from.
168          *
169          * Since orig_freezer->state == FROZEN means that @task has been
170          * frozen, so it's sufficient to check the latter condition.
171          */
172
173         if (is_task_frozen_enough(task))
174                 return -EBUSY;
175
176         freezer = cgroup_freezer(new_cgroup);
177         if (freezer->state == CGROUP_FROZEN)
178                 return -EBUSY;
179
180         return 0;
181 }
182
183 static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
184 {
185         struct freezer *freezer;
186
187         /*
188          * No lock is needed, since the task isn't on tasklist yet,
189          * so it can't be moved to another cgroup, which means the
190          * freezer won't be removed and will be valid during this
191          * function call.
192          */
193         freezer = task_freezer(task);
194
195         /*
196          * The root cgroup is non-freezable, so we can skip the
197          * following check.
198          */
199         if (!freezer->css.cgroup->parent)
200                 return;
201
202         spin_lock_irq(&freezer->lock);
203         BUG_ON(freezer->state == CGROUP_FROZEN);
204
205         /* Locking avoids race with FREEZING -> THAWED transitions. */
206         if (freezer->state == CGROUP_FREEZING)
207                 freeze_task(task, true);
208         spin_unlock_irq(&freezer->lock);
209 }
210
211 /*
212  * caller must hold freezer->lock
213  */
214 static void update_freezer_state(struct cgroup *cgroup,
215                                  struct freezer *freezer)
216 {
217         struct cgroup_iter it;
218         struct task_struct *task;
219         unsigned int nfrozen = 0, ntotal = 0;
220
221         cgroup_iter_start(cgroup, &it);
222         while ((task = cgroup_iter_next(cgroup, &it))) {
223                 ntotal++;
224                 if (is_task_frozen_enough(task))
225                         nfrozen++;
226         }
227
228         /*
229          * Transition to FROZEN when no new tasks can be added ensures
230          * that we never exist in the FROZEN state while there are unfrozen
231          * tasks.
232          */
233         if (nfrozen == ntotal)
234                 freezer->state = CGROUP_FROZEN;
235         else if (nfrozen > 0)
236                 freezer->state = CGROUP_FREEZING;
237         else
238                 freezer->state = CGROUP_THAWED;
239         cgroup_iter_end(cgroup, &it);
240 }
241
242 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
243                         struct seq_file *m)
244 {
245         struct freezer *freezer;
246         enum freezer_state state;
247
248         if (!cgroup_lock_live_group(cgroup))
249                 return -ENODEV;
250
251         freezer = cgroup_freezer(cgroup);
252         spin_lock_irq(&freezer->lock);
253         state = freezer->state;
254         if (state == CGROUP_FREEZING) {
255                 /* We change from FREEZING to FROZEN lazily if the cgroup was
256                  * only partially frozen when we exitted write. */
257                 update_freezer_state(cgroup, freezer);
258                 state = freezer->state;
259         }
260         spin_unlock_irq(&freezer->lock);
261         cgroup_unlock();
262
263         seq_puts(m, freezer_state_strs[state]);
264         seq_putc(m, '\n');
265         return 0;
266 }
267
268 static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
269 {
270         struct cgroup_iter it;
271         struct task_struct *task;
272         unsigned int num_cant_freeze_now = 0;
273
274         freezer->state = CGROUP_FREEZING;
275         cgroup_iter_start(cgroup, &it);
276         while ((task = cgroup_iter_next(cgroup, &it))) {
277                 if (!freeze_task(task, true))
278                         continue;
279                 if (is_task_frozen_enough(task))
280                         continue;
281                 if (!freezing(task) && !freezer_should_skip(task))
282                         num_cant_freeze_now++;
283         }
284         cgroup_iter_end(cgroup, &it);
285
286         return num_cant_freeze_now ? -EBUSY : 0;
287 }
288
289 static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
290 {
291         struct cgroup_iter it;
292         struct task_struct *task;
293
294         cgroup_iter_start(cgroup, &it);
295         while ((task = cgroup_iter_next(cgroup, &it))) {
296                 thaw_process(task);
297         }
298         cgroup_iter_end(cgroup, &it);
299
300         freezer->state = CGROUP_THAWED;
301 }
302
303 static 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_freezer_state(cgroup, freezer);
314         if (goal_state == freezer->state)
315                 goto out;
316
317         switch (goal_state) {
318         case CGROUP_THAWED:
319                 unfreeze_cgroup(cgroup, freezer);
320                 break;
321         case CGROUP_FROZEN:
322                 retval = try_to_freeze_cgroup(cgroup, freezer);
323                 break;
324         default:
325                 BUG();
326         }
327 out:
328         spin_unlock_irq(&freezer->lock);
329
330         return retval;
331 }
332
333 static int freezer_write(struct cgroup *cgroup,
334                          struct cftype *cft,
335                          const char *buffer)
336 {
337         int retval;
338         enum freezer_state goal_state;
339
340         if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
341                 goal_state = CGROUP_THAWED;
342         else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
343                 goal_state = CGROUP_FROZEN;
344         else
345                 return -EINVAL;
346
347         if (!cgroup_lock_live_group(cgroup))
348                 return -ENODEV;
349         retval = freezer_change_state(cgroup, goal_state);
350         cgroup_unlock();
351         return retval;
352 }
353
354 static struct cftype files[] = {
355         {
356                 .name = "state",
357                 .read_seq_string = freezer_read,
358                 .write_string = freezer_write,
359         },
360 };
361
362 static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
363 {
364         if (!cgroup->parent)
365                 return 0;
366         return cgroup_add_files(cgroup, ss, files, ARRAY_SIZE(files));
367 }
368
369 struct cgroup_subsys freezer_subsys = {
370         .name           = "freezer",
371         .create         = freezer_create,
372         .destroy        = freezer_destroy,
373         .populate       = freezer_populate,
374         .subsys_id      = freezer_subsys_id,
375         .can_attach     = freezer_can_attach,
376         .attach         = NULL,
377         .fork           = freezer_fork,
378         .exit           = NULL,
379 };