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, bool threadgroup)
 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         if (threadgroup) {
 181                 struct task_struct *c;
 182
 183                 rcu_read_lock();
 184                 list_for_each_entry_rcu(c, &task->thread_group, thread_group) {
 185                         if (is_task_frozen_enough(c)) {
 186                                 rcu_read_unlock();
 187                                 return -EBUSY;
 188                         }
 189                 }
 190                 rcu_read_unlock();
 191         }
 192
 193         return 0;
 194 }
 195
 196 static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
 197 {
 198         struct freezer *freezer;
 199
 200         /*
 201          * No lock is needed, since the task isn't on tasklist yet,
 202          * so it can't be moved to another cgroup, which means the
 203          * freezer won't be removed and will be valid during this
 204          * function call.
 205          */
 206         freezer = task_freezer(task);
 207
 208         /*
 209          * The root cgroup is non-freezable, so we can skip the
 210          * following check.
 211          */
 212         if (!freezer->css.cgroup->parent)
 213                 return;
 214
 215         spin_lock_irq(&freezer->lock);
 216         BUG_ON(freezer->state == CGROUP_FROZEN);
 217
 218         /* Locking avoids race with FREEZING -> THAWED transitions. */
 219         if (freezer->state == CGROUP_FREEZING)
 220                 freeze_task(task, true);
 221         spin_unlock_irq(&freezer->lock);
 222 }
 223
 224 /*
 225  * caller must hold freezer->lock
 226  */
 227 static void update_freezer_state(struct cgroup *cgroup,
 228                                  struct freezer *freezer)
 229 {
 230         struct cgroup_iter it;
 231         struct task_struct *task;
 232         unsigned int nfrozen = 0, ntotal = 0;
 233
 234         cgroup_iter_start(cgroup, &it);
 235         while ((task = cgroup_iter_next(cgroup, &it))) {
 236                 ntotal++;
 237                 if (is_task_frozen_enough(task))
 238                         nfrozen++;
 239         }
 240
 241         /*
 242          * Transition to FROZEN when no new tasks can be added ensures
 243          * that we never exist in the FROZEN state while there are unfrozen
 244          * tasks.
 245          */
 246         if (nfrozen == ntotal)
 247                 freezer->state = CGROUP_FROZEN;
 248         else if (nfrozen > 0)
 249                 freezer->state = CGROUP_FREEZING;
 250         else
 251                 freezer->state = CGROUP_THAWED;
 252         cgroup_iter_end(cgroup, &it);
 253 }
 254
 255 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
 256                         struct seq_file *m)
 257 {
 258         struct freezer *freezer;
 259         enum freezer_state state;
 260
 261         if (!cgroup_lock_live_group(cgroup))
 262                 return -ENODEV;
 263
 264         freezer = cgroup_freezer(cgroup);
 265         spin_lock_irq(&freezer->lock);
 266         state = freezer->state;
 267         if (state == CGROUP_FREEZING) {
 268                 /* We change from FREEZING to FROZEN lazily if the cgroup was
 269                  * only partially frozen when we exitted write. */
 270                 update_freezer_state(cgroup, freezer);
 271                 state = freezer->state;
 272         }
 273         spin_unlock_irq(&freezer->lock);
 274         cgroup_unlock();
 275
 276         seq_puts(m, freezer_state_strs[state]);
 277         seq_putc(m, '\n');
 278         return 0;
 279 }
 280
 281 static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 282 {
 283         struct cgroup_iter it;
 284         struct task_struct *task;
 285         unsigned int num_cant_freeze_now = 0;
 286
 287         freezer->state = CGROUP_FREEZING;
 288         cgroup_iter_start(cgroup, &it);
 289         while ((task = cgroup_iter_next(cgroup, &it))) {
 290                 if (!freeze_task(task, true))
 291                         continue;
 292                 if (is_task_frozen_enough(task))
 293                         continue;
 294                 if (!freezing(task) && !freezer_should_skip(task))
 295                         num_cant_freeze_now++;
 296         }
 297         cgroup_iter_end(cgroup, &it);
 298
 299         return num_cant_freeze_now ? -EBUSY : 0;
 300 }
 301
 302 static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 303 {
 304         struct cgroup_iter it;
 305         struct task_struct *task;
 306
 307         cgroup_iter_start(cgroup, &it);
 308         while ((task = cgroup_iter_next(cgroup, &it))) {
 309                 thaw_process(task);
 310         }
 311         cgroup_iter_end(cgroup, &it);
 312
 313         freezer->state = CGROUP_THAWED;
 314 }
 315
 316 static int freezer_change_state(struct cgroup *cgroup,
 317                                 enum freezer_state goal_state)
 318 {
 319         struct freezer *freezer;
 320         int retval = 0;
 321
 322         freezer = cgroup_freezer(cgroup);
 323
 324         spin_lock_irq(&freezer->lock);
 325
 326         update_freezer_state(cgroup, freezer);
 327         if (goal_state == freezer->state)
 328                 goto out;
 329
 330         switch (goal_state) {
 331         case CGROUP_THAWED:
 332                 unfreeze_cgroup(cgroup, freezer);
 333                 break;
 334         case CGROUP_FROZEN:
 335                 retval = try_to_freeze_cgroup(cgroup, freezer);
 336                 break;
 337         default:
 338                 BUG();
 339         }
 340 out:
 341         spin_unlock_irq(&freezer->lock);
 342
 343         return retval;
 344 }
 345
 346 static int freezer_write(struct cgroup *cgroup,
 347                          struct cftype *cft,
 348                          const char *buffer)
 349 {
 350         int retval;
 351         enum freezer_state goal_state;
 352
 353         if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
 354                 goal_state = CGROUP_THAWED;
 355         else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
 356                 goal_state = CGROUP_FROZEN;
 357         else
 358                 return -EINVAL;
 359
 360         if (!cgroup_lock_live_group(cgroup))
 361                 return -ENODEV;
 362         retval = freezer_change_state(cgroup, goal_state);
 363         cgroup_unlock();
 364         return retval;
 365 }
 366
 367 static struct cftype files[] = {
 368         {
 369                 .name = "state",
 370                 .read_seq_string = freezer_read,
 371                 .write_string = freezer_write,
 372         },
 373 };
 374
 375 static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
 376 {
 377         if (!cgroup->parent)
 378                 return 0;
 379         return cgroup_add_files(cgroup, ss, files, ARRAY_SIZE(files));
 380 }
 381
 382 struct cgroup_subsys freezer_subsys = {
 383         .name           = "freezer",
 384         .create         = freezer_create,
 385         .destroy        = freezer_destroy,
 386         .populate       = freezer_populate,
 387         .subsys_id      = freezer_subsys_id,
 388         .can_attach     = freezer_can_attach,
 389         .attach         = NULL,
 390         .fork           = freezer_fork,
 391         .exit           = NULL,
 392 };