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 #include <linux/mutex.h>
25 
26 /*
27  * A cgroup is freezing if any FREEZING flags are set.  FREEZING_SELF is
28  * set if "FROZEN" is written to freezer.state cgroupfs file, and cleared
29  * for "THAWED".  FREEZING_PARENT is set if the parent freezer is FREEZING
30  * for whatever reason.  IOW, a cgroup has FREEZING_PARENT set if one of
31  * its ancestors has FREEZING_SELF set.
32  */
33 enum freezer_state_flags {
34 	CGROUP_FREEZER_ONLINE	= (1 << 0), /* freezer is fully online */
35 	CGROUP_FREEZING_SELF	= (1 << 1), /* this freezer is freezing */
36 	CGROUP_FREEZING_PARENT	= (1 << 2), /* the parent freezer is freezing */
37 	CGROUP_FROZEN		= (1 << 3), /* this and its descendants frozen */
38 
39 	/* mask for all FREEZING flags */
40 	CGROUP_FREEZING		= CGROUP_FREEZING_SELF | CGROUP_FREEZING_PARENT,
41 };
42 
43 struct freezer {
44 	struct cgroup_subsys_state	css;
45 	unsigned int			state;
46 };
47 
48 static DEFINE_MUTEX(freezer_mutex);
49 
50 static inline struct freezer *css_freezer(struct cgroup_subsys_state *css)
51 {
52 	return css ? container_of(css, struct freezer, css) : NULL;
53 }
54 
55 static inline struct freezer *task_freezer(struct task_struct *task)
56 {
57 	return css_freezer(task_css(task, freezer_cgrp_id));
58 }
59 
60 static struct freezer *parent_freezer(struct freezer *freezer)
61 {
62 	return css_freezer(freezer->css.parent);
63 }
64 
65 bool cgroup_freezing(struct task_struct *task)
66 {
67 	bool ret;
68 
69 	rcu_read_lock();
70 	ret = task_freezer(task)->state & CGROUP_FREEZING;
71 	rcu_read_unlock();
72 
73 	return ret;
74 }
75 
76 static const char *freezer_state_strs(unsigned int state)
77 {
78 	if (state & CGROUP_FROZEN)
79 		return "FROZEN";
80 	if (state & CGROUP_FREEZING)
81 		return "FREEZING";
82 	return "THAWED";
83 };
84 
85 static struct cgroup_subsys_state *
86 freezer_css_alloc(struct cgroup_subsys_state *parent_css)
87 {
88 	struct freezer *freezer;
89 
90 	freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
91 	if (!freezer)
92 		return ERR_PTR(-ENOMEM);
93 
94 	return &freezer->css;
95 }
96 
97 /**
98  * freezer_css_online - commit creation of a freezer css
99  * @css: css being created
100  *
101  * We're committing to creation of @css.  Mark it online and inherit
102  * parent's freezing state while holding both parent's and our
103  * freezer->lock.
104  */
105 static int freezer_css_online(struct cgroup_subsys_state *css)
106 {
107 	struct freezer *freezer = css_freezer(css);
108 	struct freezer *parent = parent_freezer(freezer);
109 
110 	mutex_lock(&freezer_mutex);
111 
112 	freezer->state |= CGROUP_FREEZER_ONLINE;
113 
114 	if (parent && (parent->state & CGROUP_FREEZING)) {
115 		freezer->state |= CGROUP_FREEZING_PARENT | CGROUP_FROZEN;
116 		atomic_inc(&system_freezing_cnt);
117 	}
118 
119 	mutex_unlock(&freezer_mutex);
120 	return 0;
121 }
122 
123 /**
124  * freezer_css_offline - initiate destruction of a freezer css
125  * @css: css being destroyed
126  *
127  * @css is going away.  Mark it dead and decrement system_freezing_count if
128  * it was holding one.
129  */
130 static void freezer_css_offline(struct cgroup_subsys_state *css)
131 {
132 	struct freezer *freezer = css_freezer(css);
133 
134 	mutex_lock(&freezer_mutex);
135 
136 	if (freezer->state & CGROUP_FREEZING)
137 		atomic_dec(&system_freezing_cnt);
138 
139 	freezer->state = 0;
140 
141 	mutex_unlock(&freezer_mutex);
142 }
143 
144 static void freezer_css_free(struct cgroup_subsys_state *css)
145 {
146 	kfree(css_freezer(css));
147 }
148 
149 /*
150  * Tasks can be migrated into a different freezer anytime regardless of its
151  * current state.  freezer_attach() is responsible for making new tasks
152  * conform to the current state.
153  *
154  * Freezer state changes and task migration are synchronized via
155  * @freezer->lock.  freezer_attach() makes the new tasks conform to the
156  * current state and all following state changes can see the new tasks.
157  */
158 static void freezer_attach(struct cgroup_taskset *tset)
159 {
160 	struct task_struct *task;
161 	struct cgroup_subsys_state *new_css;
162 
163 	mutex_lock(&freezer_mutex);
164 
165 	/*
166 	 * Make the new tasks conform to the current state of @new_css.
167 	 * For simplicity, when migrating any task to a FROZEN cgroup, we
168 	 * revert it to FREEZING and let update_if_frozen() determine the
169 	 * correct state later.
170 	 *
171 	 * Tasks in @tset are on @new_css but may not conform to its
172 	 * current state before executing the following - !frozen tasks may
173 	 * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
174 	 */
175 	cgroup_taskset_for_each(task, new_css, tset) {
176 		struct freezer *freezer = css_freezer(new_css);
177 
178 		if (!(freezer->state & CGROUP_FREEZING)) {
179 			__thaw_task(task);
180 		} else {
181 			freeze_task(task);
182 			/* clear FROZEN and propagate upwards */
183 			while (freezer && (freezer->state & CGROUP_FROZEN)) {
184 				freezer->state &= ~CGROUP_FROZEN;
185 				freezer = parent_freezer(freezer);
186 			}
187 		}
188 	}
189 
190 	mutex_unlock(&freezer_mutex);
191 }
192 
193 /**
194  * freezer_fork - cgroup post fork callback
195  * @task: a task which has just been forked
196  *
197  * @task has just been created and should conform to the current state of
198  * the cgroup_freezer it belongs to.  This function may race against
199  * freezer_attach().  Losing to freezer_attach() means that we don't have
200  * to do anything as freezer_attach() will put @task into the appropriate
201  * state.
202  */
203 static void freezer_fork(struct task_struct *task)
204 {
205 	struct freezer *freezer;
206 
207 	/*
208 	 * The root cgroup is non-freezable, so we can skip locking the
209 	 * freezer.  This is safe regardless of race with task migration.
210 	 * If we didn't race or won, skipping is obviously the right thing
211 	 * to do.  If we lost and root is the new cgroup, noop is still the
212 	 * right thing to do.
213 	 */
214 	if (task_css_is_root(task, freezer_cgrp_id))
215 		return;
216 
217 	mutex_lock(&freezer_mutex);
218 	rcu_read_lock();
219 
220 	freezer = task_freezer(task);
221 	if (freezer->state & CGROUP_FREEZING)
222 		freeze_task(task);
223 
224 	rcu_read_unlock();
225 	mutex_unlock(&freezer_mutex);
226 }
227 
228 /**
229  * update_if_frozen - update whether a cgroup finished freezing
230  * @css: css of interest
231  *
232  * Once FREEZING is initiated, transition to FROZEN is lazily updated by
233  * calling this function.  If the current state is FREEZING but not FROZEN,
234  * this function checks whether all tasks of this cgroup and the descendant
235  * cgroups finished freezing and, if so, sets FROZEN.
236  *
237  * The caller is responsible for grabbing RCU read lock and calling
238  * update_if_frozen() on all descendants prior to invoking this function.
239  *
240  * Task states and freezer state might disagree while tasks are being
241  * migrated into or out of @css, so we can't verify task states against
242  * @freezer state here.  See freezer_attach() for details.
243  */
244 static void update_if_frozen(struct cgroup_subsys_state *css)
245 {
246 	struct freezer *freezer = css_freezer(css);
247 	struct cgroup_subsys_state *pos;
248 	struct css_task_iter it;
249 	struct task_struct *task;
250 
251 	lockdep_assert_held(&freezer_mutex);
252 
253 	if (!(freezer->state & CGROUP_FREEZING) ||
254 	    (freezer->state & CGROUP_FROZEN))
255 		return;
256 
257 	/* are all (live) children frozen? */
258 	rcu_read_lock();
259 	css_for_each_child(pos, css) {
260 		struct freezer *child = css_freezer(pos);
261 
262 		if ((child->state & CGROUP_FREEZER_ONLINE) &&
263 		    !(child->state & CGROUP_FROZEN)) {
264 			rcu_read_unlock();
265 			return;
266 		}
267 	}
268 	rcu_read_unlock();
269 
270 	/* are all tasks frozen? */
271 	css_task_iter_start(css, 0, &it);
272 
273 	while ((task = css_task_iter_next(&it))) {
274 		if (freezing(task)) {
275 			/*
276 			 * freezer_should_skip() indicates that the task
277 			 * should be skipped when determining freezing
278 			 * completion.  Consider it frozen in addition to
279 			 * the usual frozen condition.
280 			 */
281 			if (!frozen(task) && !freezer_should_skip(task))
282 				goto out_iter_end;
283 		}
284 	}
285 
286 	freezer->state |= CGROUP_FROZEN;
287 out_iter_end:
288 	css_task_iter_end(&it);
289 }
290 
291 static int freezer_read(struct seq_file *m, void *v)
292 {
293 	struct cgroup_subsys_state *css = seq_css(m), *pos;
294 
295 	mutex_lock(&freezer_mutex);
296 	rcu_read_lock();
297 
298 	/* update states bottom-up */
299 	css_for_each_descendant_post(pos, css) {
300 		if (!css_tryget_online(pos))
301 			continue;
302 		rcu_read_unlock();
303 
304 		update_if_frozen(pos);
305 
306 		rcu_read_lock();
307 		css_put(pos);
308 	}
309 
310 	rcu_read_unlock();
311 	mutex_unlock(&freezer_mutex);
312 
313 	seq_puts(m, freezer_state_strs(css_freezer(css)->state));
314 	seq_putc(m, '\n');
315 	return 0;
316 }
317 
318 static void freeze_cgroup(struct freezer *freezer)
319 {
320 	struct css_task_iter it;
321 	struct task_struct *task;
322 
323 	css_task_iter_start(&freezer->css, 0, &it);
324 	while ((task = css_task_iter_next(&it)))
325 		freeze_task(task);
326 	css_task_iter_end(&it);
327 }
328 
329 static void unfreeze_cgroup(struct freezer *freezer)
330 {
331 	struct css_task_iter it;
332 	struct task_struct *task;
333 
334 	css_task_iter_start(&freezer->css, 0, &it);
335 	while ((task = css_task_iter_next(&it)))
336 		__thaw_task(task);
337 	css_task_iter_end(&it);
338 }
339 
340 /**
341  * freezer_apply_state - apply state change to a single cgroup_freezer
342  * @freezer: freezer to apply state change to
343  * @freeze: whether to freeze or unfreeze
344  * @state: CGROUP_FREEZING_* flag to set or clear
345  *
346  * Set or clear @state on @cgroup according to @freeze, and perform
347  * freezing or thawing as necessary.
348  */
349 static void freezer_apply_state(struct freezer *freezer, bool freeze,
350 				unsigned int state)
351 {
352 	/* also synchronizes against task migration, see freezer_attach() */
353 	lockdep_assert_held(&freezer_mutex);
354 
355 	if (!(freezer->state & CGROUP_FREEZER_ONLINE))
356 		return;
357 
358 	if (freeze) {
359 		if (!(freezer->state & CGROUP_FREEZING))
360 			atomic_inc(&system_freezing_cnt);
361 		freezer->state |= state;
362 		freeze_cgroup(freezer);
363 	} else {
364 		bool was_freezing = freezer->state & CGROUP_FREEZING;
365 
366 		freezer->state &= ~state;
367 
368 		if (!(freezer->state & CGROUP_FREEZING)) {
369 			if (was_freezing)
370 				atomic_dec(&system_freezing_cnt);
371 			freezer->state &= ~CGROUP_FROZEN;
372 			unfreeze_cgroup(freezer);
373 		}
374 	}
375 }
376 
377 /**
378  * freezer_change_state - change the freezing state of a cgroup_freezer
379  * @freezer: freezer of interest
380  * @freeze: whether to freeze or thaw
381  *
382  * Freeze or thaw @freezer according to @freeze.  The operations are
383  * recursive - all descendants of @freezer will be affected.
384  */
385 static void freezer_change_state(struct freezer *freezer, bool freeze)
386 {
387 	struct cgroup_subsys_state *pos;
388 
389 	/*
390 	 * Update all its descendants in pre-order traversal.  Each
391 	 * descendant will try to inherit its parent's FREEZING state as
392 	 * CGROUP_FREEZING_PARENT.
393 	 */
394 	mutex_lock(&freezer_mutex);
395 	rcu_read_lock();
396 	css_for_each_descendant_pre(pos, &freezer->css) {
397 		struct freezer *pos_f = css_freezer(pos);
398 		struct freezer *parent = parent_freezer(pos_f);
399 
400 		if (!css_tryget_online(pos))
401 			continue;
402 		rcu_read_unlock();
403 
404 		if (pos_f == freezer)
405 			freezer_apply_state(pos_f, freeze,
406 					    CGROUP_FREEZING_SELF);
407 		else
408 			freezer_apply_state(pos_f,
409 					    parent->state & CGROUP_FREEZING,
410 					    CGROUP_FREEZING_PARENT);
411 
412 		rcu_read_lock();
413 		css_put(pos);
414 	}
415 	rcu_read_unlock();
416 	mutex_unlock(&freezer_mutex);
417 }
418 
419 static ssize_t freezer_write(struct kernfs_open_file *of,
420 			     char *buf, size_t nbytes, loff_t off)
421 {
422 	bool freeze;
423 
424 	buf = strstrip(buf);
425 
426 	if (strcmp(buf, freezer_state_strs(0)) == 0)
427 		freeze = false;
428 	else if (strcmp(buf, freezer_state_strs(CGROUP_FROZEN)) == 0)
429 		freeze = true;
430 	else
431 		return -EINVAL;
432 
433 	freezer_change_state(css_freezer(of_css(of)), freeze);
434 	return nbytes;
435 }
436 
437 static u64 freezer_self_freezing_read(struct cgroup_subsys_state *css,
438 				      struct cftype *cft)
439 {
440 	struct freezer *freezer = css_freezer(css);
441 
442 	return (bool)(freezer->state & CGROUP_FREEZING_SELF);
443 }
444 
445 static u64 freezer_parent_freezing_read(struct cgroup_subsys_state *css,
446 					struct cftype *cft)
447 {
448 	struct freezer *freezer = css_freezer(css);
449 
450 	return (bool)(freezer->state & CGROUP_FREEZING_PARENT);
451 }
452 
453 static struct cftype files[] = {
454 	{
455 		.name = "state",
456 		.flags = CFTYPE_NOT_ON_ROOT,
457 		.seq_show = freezer_read,
458 		.write = freezer_write,
459 	},
460 	{
461 		.name = "self_freezing",
462 		.flags = CFTYPE_NOT_ON_ROOT,
463 		.read_u64 = freezer_self_freezing_read,
464 	},
465 	{
466 		.name = "parent_freezing",
467 		.flags = CFTYPE_NOT_ON_ROOT,
468 		.read_u64 = freezer_parent_freezing_read,
469 	},
470 	{ }	/* terminate */
471 };
472 
473 struct cgroup_subsys freezer_cgrp_subsys = {
474 	.css_alloc	= freezer_css_alloc,
475 	.css_online	= freezer_css_online,
476 	.css_offline	= freezer_css_offline,
477 	.css_free	= freezer_css_free,
478 	.attach		= freezer_attach,
479 	.fork		= freezer_fork,
480 	.legacy_cftypes	= files,
481 };
482