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#include <linux/cpu.h>
26
27/*
28 * A cgroup is freezing if any FREEZING flags are set.  FREEZING_SELF is
29 * set if "FROZEN" is written to freezer.state cgroupfs file, and cleared
30 * for "THAWED".  FREEZING_PARENT is set if the parent freezer is FREEZING
31 * for whatever reason.  IOW, a cgroup has FREEZING_PARENT set if one of
32 * its ancestors has FREEZING_SELF set.
33 */
34enum freezer_state_flags {
35	CGROUP_FREEZER_ONLINE	= (1 << 0), /* freezer is fully online */
36	CGROUP_FREEZING_SELF	= (1 << 1), /* this freezer is freezing */
37	CGROUP_FREEZING_PARENT	= (1 << 2), /* the parent freezer is freezing */
38	CGROUP_FROZEN		= (1 << 3), /* this and its descendants frozen */
39
40	/* mask for all FREEZING flags */
41	CGROUP_FREEZING		= CGROUP_FREEZING_SELF | CGROUP_FREEZING_PARENT,
42};
43
44struct freezer {
45	struct cgroup_subsys_state	css;
46	unsigned int			state;
47};
48
49static DEFINE_MUTEX(freezer_mutex);
50
51static inline struct freezer *css_freezer(struct cgroup_subsys_state *css)
52{
53	return css ? container_of(css, struct freezer, css) : NULL;
54}
55
56static inline struct freezer *task_freezer(struct task_struct *task)
57{
58	return css_freezer(task_css(task, freezer_cgrp_id));
59}
60
61static struct freezer *parent_freezer(struct freezer *freezer)
62{
63	return css_freezer(freezer->css.parent);
64}
65
66bool cgroup_freezing(struct task_struct *task)
67{
68	bool ret;
69	unsigned int state;
70
71	rcu_read_lock();
72	/* Check if the cgroup is still FREEZING, but not FROZEN. The extra
73	 * !FROZEN check is required, because the FREEZING bit is not cleared
74	 * when the state FROZEN is reached.
75	 */
76	state = task_freezer(task)->state;
77	ret = (state & CGROUP_FREEZING) && !(state & CGROUP_FROZEN);
78	rcu_read_unlock();
79
80	return ret;
81}
82
83static const char *freezer_state_strs(unsigned int state)
84{
85	if (state & CGROUP_FROZEN)
86		return "FROZEN";
87	if (state & CGROUP_FREEZING)
88		return "FREEZING";
89	return "THAWED";
90};
91
92static struct cgroup_subsys_state *
93freezer_css_alloc(struct cgroup_subsys_state *parent_css)
94{
95	struct freezer *freezer;
96
97	freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
98	if (!freezer)
99		return ERR_PTR(-ENOMEM);
100
101	return &freezer->css;
102}
103
104/**
105 * freezer_css_online - commit creation of a freezer css
106 * @css: css being created
107 *
108 * We're committing to creation of @css.  Mark it online and inherit
109 * parent's freezing state while holding both parent's and our
110 * freezer->lock.
111 */
112static int freezer_css_online(struct cgroup_subsys_state *css)
113{
114	struct freezer *freezer = css_freezer(css);
115	struct freezer *parent = parent_freezer(freezer);
116
117	cpus_read_lock();
118	mutex_lock(&freezer_mutex);
119
120	freezer->state |= CGROUP_FREEZER_ONLINE;
121
122	if (parent && (parent->state & CGROUP_FREEZING)) {
123		freezer->state |= CGROUP_FREEZING_PARENT | CGROUP_FROZEN;
124		static_branch_inc_cpuslocked(&freezer_active);
125	}
126
127	mutex_unlock(&freezer_mutex);
128	cpus_read_unlock();
129	return 0;
130}
131
132/**
133 * freezer_css_offline - initiate destruction of a freezer css
134 * @css: css being destroyed
135 *
136 * @css is going away.  Mark it dead and decrement system_freezing_count if
137 * it was holding one.
138 */
139static void freezer_css_offline(struct cgroup_subsys_state *css)
140{
141	struct freezer *freezer = css_freezer(css);
142
143	cpus_read_lock();
144	mutex_lock(&freezer_mutex);
145
146	if (freezer->state & CGROUP_FREEZING)
147		static_branch_dec_cpuslocked(&freezer_active);
148
149	freezer->state = 0;
150
151	mutex_unlock(&freezer_mutex);
152	cpus_read_unlock();
153}
154
155static void freezer_css_free(struct cgroup_subsys_state *css)
156{
157	kfree(css_freezer(css));
158}
159
160/*
161 * Tasks can be migrated into a different freezer anytime regardless of its
162 * current state.  freezer_attach() is responsible for making new tasks
163 * conform to the current state.
164 *
165 * Freezer state changes and task migration are synchronized via
166 * @freezer->lock.  freezer_attach() makes the new tasks conform to the
167 * current state and all following state changes can see the new tasks.
168 */
169static void freezer_attach(struct cgroup_taskset *tset)
170{
171	struct task_struct *task;
172	struct cgroup_subsys_state *new_css;
173
174	mutex_lock(&freezer_mutex);
175
176	/*
177	 * Make the new tasks conform to the current state of @new_css.
178	 * For simplicity, when migrating any task to a FROZEN cgroup, we
179	 * revert it to FREEZING and let update_if_frozen() determine the
180	 * correct state later.
181	 *
182	 * Tasks in @tset are on @new_css but may not conform to its
183	 * current state before executing the following - !frozen tasks may
184	 * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
185	 */
186	cgroup_taskset_for_each(task, new_css, tset) {
187		struct freezer *freezer = css_freezer(new_css);
188
189		if (!(freezer->state & CGROUP_FREEZING)) {
190			__thaw_task(task);
191		} else {
192			freeze_task(task);
193
194			/* clear FROZEN and propagate upwards */
195			while (freezer && (freezer->state & CGROUP_FROZEN)) {
196				freezer->state &= ~CGROUP_FROZEN;
197				freezer = parent_freezer(freezer);
198			}
199		}
200	}
201
202	mutex_unlock(&freezer_mutex);
203}
204
205/**
206 * freezer_fork - cgroup post fork callback
207 * @task: a task which has just been forked
208 *
209 * @task has just been created and should conform to the current state of
210 * the cgroup_freezer it belongs to.  This function may race against
211 * freezer_attach().  Losing to freezer_attach() means that we don't have
212 * to do anything as freezer_attach() will put @task into the appropriate
213 * state.
214 */
215static void freezer_fork(struct task_struct *task)
216{
217	struct freezer *freezer;
218
219	/*
220	 * The root cgroup is non-freezable, so we can skip locking the
221	 * freezer.  This is safe regardless of race with task migration.
222	 * If we didn't race or won, skipping is obviously the right thing
223	 * to do.  If we lost and root is the new cgroup, noop is still the
224	 * right thing to do.
225	 */
226	if (task_css_is_root(task, freezer_cgrp_id))
227		return;
228
229	mutex_lock(&freezer_mutex);
230	rcu_read_lock();
231
232	freezer = task_freezer(task);
233	if (freezer->state & CGROUP_FREEZING)
234		freeze_task(task);
235
236	rcu_read_unlock();
237	mutex_unlock(&freezer_mutex);
238}
239
240/**
241 * update_if_frozen - update whether a cgroup finished freezing
242 * @css: css of interest
243 *
244 * Once FREEZING is initiated, transition to FROZEN is lazily updated by
245 * calling this function.  If the current state is FREEZING but not FROZEN,
246 * this function checks whether all tasks of this cgroup and the descendant
247 * cgroups finished freezing and, if so, sets FROZEN.
248 *
249 * The caller is responsible for grabbing RCU read lock and calling
250 * update_if_frozen() on all descendants prior to invoking this function.
251 *
252 * Task states and freezer state might disagree while tasks are being
253 * migrated into or out of @css, so we can't verify task states against
254 * @freezer state here.  See freezer_attach() for details.
255 */
256static void update_if_frozen(struct cgroup_subsys_state *css)
257{
258	struct freezer *freezer = css_freezer(css);
259	struct cgroup_subsys_state *pos;
260	struct css_task_iter it;
261	struct task_struct *task;
262
263	lockdep_assert_held(&freezer_mutex);
264
265	if (!(freezer->state & CGROUP_FREEZING) ||
266	    (freezer->state & CGROUP_FROZEN))
267		return;
268
269	/* are all (live) children frozen? */
270	rcu_read_lock();
271	css_for_each_child(pos, css) {
272		struct freezer *child = css_freezer(pos);
273
274		if ((child->state & CGROUP_FREEZER_ONLINE) &&
275		    !(child->state & CGROUP_FROZEN)) {
276			rcu_read_unlock();
277			return;
278		}
279	}
280	rcu_read_unlock();
281
282	/* are all tasks frozen? */
283	css_task_iter_start(css, 0, &it);
284
285	while ((task = css_task_iter_next(&it))) {
286		if (freezing(task) && !frozen(task))
287			goto out_iter_end;
288	}
289
290	freezer->state |= CGROUP_FROZEN;
291out_iter_end:
292	css_task_iter_end(&it);
293}
294
295static int freezer_read(struct seq_file *m, void *v)
296{
297	struct cgroup_subsys_state *css = seq_css(m), *pos;
298
299	mutex_lock(&freezer_mutex);
300	rcu_read_lock();
301
302	/* update states bottom-up */
303	css_for_each_descendant_post(pos, css) {
304		if (!css_tryget_online(pos))
305			continue;
306		rcu_read_unlock();
307
308		update_if_frozen(pos);
309
310		rcu_read_lock();
311		css_put(pos);
312	}
313
314	rcu_read_unlock();
315	mutex_unlock(&freezer_mutex);
316
317	seq_puts(m, freezer_state_strs(css_freezer(css)->state));
318	seq_putc(m, '\n');
319	return 0;
320}
321
322static void freeze_cgroup(struct freezer *freezer)
323{
324	struct css_task_iter it;
325	struct task_struct *task;
326
327	css_task_iter_start(&freezer->css, 0, &it);
328	while ((task = css_task_iter_next(&it)))
329		freeze_task(task);
330	css_task_iter_end(&it);
331}
332
333static void unfreeze_cgroup(struct freezer *freezer)
334{
335	struct css_task_iter it;
336	struct task_struct *task;
337
338	css_task_iter_start(&freezer->css, 0, &it);
339	while ((task = css_task_iter_next(&it)))
340		__thaw_task(task);
341	css_task_iter_end(&it);
342}
343
344/**
345 * freezer_apply_state - apply state change to a single cgroup_freezer
346 * @freezer: freezer to apply state change to
347 * @freeze: whether to freeze or unfreeze
348 * @state: CGROUP_FREEZING_* flag to set or clear
349 *
350 * Set or clear @state on @cgroup according to @freeze, and perform
351 * freezing or thawing as necessary.
352 */
353static void freezer_apply_state(struct freezer *freezer, bool freeze,
354				unsigned int state)
355{
356	/* also synchronizes against task migration, see freezer_attach() */
357	lockdep_assert_held(&freezer_mutex);
358
359	if (!(freezer->state & CGROUP_FREEZER_ONLINE))
360		return;
361
362	if (freeze) {
363		if (!(freezer->state & CGROUP_FREEZING))
364			static_branch_inc_cpuslocked(&freezer_active);
365		freezer->state |= state;
366		freeze_cgroup(freezer);
367	} else {
368		bool was_freezing = freezer->state & CGROUP_FREEZING;
369
370		freezer->state &= ~state;
371
372		if (!(freezer->state & CGROUP_FREEZING)) {
373			freezer->state &= ~CGROUP_FROZEN;
374			if (was_freezing)
375				static_branch_dec_cpuslocked(&freezer_active);
376			unfreeze_cgroup(freezer);
377		}
378	}
379}
380
381/**
382 * freezer_change_state - change the freezing state of a cgroup_freezer
383 * @freezer: freezer of interest
384 * @freeze: whether to freeze or thaw
385 *
386 * Freeze or thaw @freezer according to @freeze.  The operations are
387 * recursive - all descendants of @freezer will be affected.
388 */
389static void freezer_change_state(struct freezer *freezer, bool freeze)
390{
391	struct cgroup_subsys_state *pos;
392
393	cpus_read_lock();
394	/*
395	 * Update all its descendants in pre-order traversal.  Each
396	 * descendant will try to inherit its parent's FREEZING state as
397	 * CGROUP_FREEZING_PARENT.
398	 */
399	mutex_lock(&freezer_mutex);
400	rcu_read_lock();
401	css_for_each_descendant_pre(pos, &freezer->css) {
402		struct freezer *pos_f = css_freezer(pos);
403		struct freezer *parent = parent_freezer(pos_f);
404
405		if (!css_tryget_online(pos))
406			continue;
407		rcu_read_unlock();
408
409		if (pos_f == freezer)
410			freezer_apply_state(pos_f, freeze,
411					    CGROUP_FREEZING_SELF);
412		else
413			freezer_apply_state(pos_f,
414					    parent->state & CGROUP_FREEZING,
415					    CGROUP_FREEZING_PARENT);
416
417		rcu_read_lock();
418		css_put(pos);
419	}
420	rcu_read_unlock();
421	mutex_unlock(&freezer_mutex);
422	cpus_read_unlock();
423}
424
425static ssize_t freezer_write(struct kernfs_open_file *of,
426			     char *buf, size_t nbytes, loff_t off)
427{
428	bool freeze;
429
430	buf = strstrip(buf);
431
432	if (strcmp(buf, freezer_state_strs(0)) == 0)
433		freeze = false;
434	else if (strcmp(buf, freezer_state_strs(CGROUP_FROZEN)) == 0)
435		freeze = true;
436	else
437		return -EINVAL;
438
439	freezer_change_state(css_freezer(of_css(of)), freeze);
440	return nbytes;
441}
442
443static u64 freezer_self_freezing_read(struct cgroup_subsys_state *css,
444				      struct cftype *cft)
445{
446	struct freezer *freezer = css_freezer(css);
447
448	return (bool)(freezer->state & CGROUP_FREEZING_SELF);
449}
450
451static u64 freezer_parent_freezing_read(struct cgroup_subsys_state *css,
452					struct cftype *cft)
453{
454	struct freezer *freezer = css_freezer(css);
455
456	return (bool)(freezer->state & CGROUP_FREEZING_PARENT);
457}
458
459static struct cftype files[] = {
460	{
461		.name = "state",
462		.flags = CFTYPE_NOT_ON_ROOT,
463		.seq_show = freezer_read,
464		.write = freezer_write,
465	},
466	{
467		.name = "self_freezing",
468		.flags = CFTYPE_NOT_ON_ROOT,
469		.read_u64 = freezer_self_freezing_read,
470	},
471	{
472		.name = "parent_freezing",
473		.flags = CFTYPE_NOT_ON_ROOT,
474		.read_u64 = freezer_parent_freezing_read,
475	},
476	{ }	/* terminate */
477};
478
479struct cgroup_subsys freezer_cgrp_subsys = {
480	.css_alloc	= freezer_css_alloc,
481	.css_online	= freezer_css_online,
482	.css_offline	= freezer_css_offline,
483	.css_free	= freezer_css_free,
484	.attach		= freezer_attach,
485	.fork		= freezer_fork,
486	.legacy_cftypes	= files,
487};
488