freezer_cg: remove task_lock from freezer_fork()
[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         spin_lock_irq(&freezer->lock);
196         BUG_ON(freezer->state == CGROUP_FROZEN);
197
198         /* Locking avoids race with FREEZING -> THAWED transitions. */
199         if (freezer->state == CGROUP_FREEZING)
200                 freeze_task(task, true);
201         spin_unlock_irq(&freezer->lock);
202 }
203
204 /*
205  * caller must hold freezer->lock
206  */
207 static void update_freezer_state(struct cgroup *cgroup,
208                                  struct freezer *freezer)
209 {
210         struct cgroup_iter it;
211         struct task_struct *task;
212         unsigned int nfrozen = 0, ntotal = 0;
213
214         cgroup_iter_start(cgroup, &it);
215         while ((task = cgroup_iter_next(cgroup, &it))) {
216                 ntotal++;
217                 if (is_task_frozen_enough(task))
218                         nfrozen++;
219         }
220
221         /*
222          * Transition to FROZEN when no new tasks can be added ensures
223          * that we never exist in the FROZEN state while there are unfrozen
224          * tasks.
225          */
226         if (nfrozen == ntotal)
227                 freezer->state = CGROUP_FROZEN;
228         else if (nfrozen > 0)
229                 freezer->state = CGROUP_FREEZING;
230         else
231                 freezer->state = CGROUP_THAWED;
232         cgroup_iter_end(cgroup, &it);
233 }
234
235 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
236                         struct seq_file *m)
237 {
238         struct freezer *freezer;
239         enum freezer_state state;
240
241         if (!cgroup_lock_live_group(cgroup))
242                 return -ENODEV;
243
244         freezer = cgroup_freezer(cgroup);
245         spin_lock_irq(&freezer->lock);
246         state = freezer->state;
247         if (state == CGROUP_FREEZING) {
248                 /* We change from FREEZING to FROZEN lazily if the cgroup was
249                  * only partially frozen when we exitted write. */
250                 update_freezer_state(cgroup, freezer);
251                 state = freezer->state;
252         }
253         spin_unlock_irq(&freezer->lock);
254         cgroup_unlock();
255
256         seq_puts(m, freezer_state_strs[state]);
257         seq_putc(m, '\n');
258         return 0;
259 }
260
261 static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
262 {
263         struct cgroup_iter it;
264         struct task_struct *task;
265         unsigned int num_cant_freeze_now = 0;
266
267         freezer->state = CGROUP_FREEZING;
268         cgroup_iter_start(cgroup, &it);
269         while ((task = cgroup_iter_next(cgroup, &it))) {
270                 if (!freeze_task(task, true))
271                         continue;
272                 if (is_task_frozen_enough(task))
273                         continue;
274                 if (!freezing(task) && !freezer_should_skip(task))
275                         num_cant_freeze_now++;
276         }
277         cgroup_iter_end(cgroup, &it);
278
279         return num_cant_freeze_now ? -EBUSY : 0;
280 }
281
282 static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
283 {
284         struct cgroup_iter it;
285         struct task_struct *task;
286
287         cgroup_iter_start(cgroup, &it);
288         while ((task = cgroup_iter_next(cgroup, &it))) {
289                 thaw_process(task);
290         }
291         cgroup_iter_end(cgroup, &it);
292
293         freezer->state = CGROUP_THAWED;
294 }
295
296 static int freezer_change_state(struct cgroup *cgroup,
297                                 enum freezer_state goal_state)
298 {
299         struct freezer *freezer;
300         int retval = 0;
301
302         freezer = cgroup_freezer(cgroup);
303
304         spin_lock_irq(&freezer->lock);
305
306         update_freezer_state(cgroup, freezer);
307         if (goal_state == freezer->state)
308                 goto out;
309
310         switch (goal_state) {
311         case CGROUP_THAWED:
312                 unfreeze_cgroup(cgroup, freezer);
313                 break;
314         case CGROUP_FROZEN:
315                 retval = try_to_freeze_cgroup(cgroup, freezer);
316                 break;
317         default:
318                 BUG();
319         }
320 out:
321         spin_unlock_irq(&freezer->lock);
322
323         return retval;
324 }
325
326 static int freezer_write(struct cgroup *cgroup,
327                          struct cftype *cft,
328                          const char *buffer)
329 {
330         int retval;
331         enum freezer_state goal_state;
332
333         if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
334                 goal_state = CGROUP_THAWED;
335         else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
336                 goal_state = CGROUP_FROZEN;
337         else
338                 return -EIO;
339
340         if (!cgroup_lock_live_group(cgroup))
341                 return -ENODEV;
342         retval = freezer_change_state(cgroup, goal_state);
343         cgroup_unlock();
344         return retval;
345 }
346
347 static struct cftype files[] = {
348         {
349                 .name = "state",
350                 .read_seq_string = freezer_read,
351                 .write_string = freezer_write,
352         },
353 };
354
355 static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
356 {
357         return cgroup_add_files(cgroup, ss, files, ARRAY_SIZE(files));
358 }
359
360 struct cgroup_subsys freezer_subsys = {
361         .name           = "freezer",
362         .create         = freezer_create,
363         .destroy        = freezer_destroy,
364         .populate       = freezer_populate,
365         .subsys_id      = freezer_subsys_id,
366         .can_attach     = freezer_can_attach,
367         .attach         = NULL,
368         .fork           = freezer_fork,
369         .exit           = NULL,
370 };