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         int retval;
 166
 167         /* Anything frozen can't move or be moved to/from */
 168
 169         if (is_task_frozen_enough(task))
 170                 return -EBUSY;
 171
 172         freezer = cgroup_freezer(new_cgroup);
 173         if (freezer->state == CGROUP_FROZEN)
 174                 return -EBUSY;
 175
 176         retval = 0;
 177         task_lock(task);
 178         freezer = task_freezer(task);
 179         if (freezer->state == CGROUP_FROZEN)
 180                 retval = -EBUSY;
 181         task_unlock(task);
 182         return retval;
 183 }
 184
 185 static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
 186 {
 187         struct freezer *freezer;
 188
 189         task_lock(task);
 190         freezer = task_freezer(task);
 191         task_unlock(task);
 192
 193         BUG_ON(freezer->state == CGROUP_FROZEN);
 194         spin_lock_irq(&freezer->lock);
 195         /* Locking avoids race with FREEZING -> THAWED transitions. */
 196         if (freezer->state == CGROUP_FREEZING)
 197                 freeze_task(task, true);
 198         spin_unlock_irq(&freezer->lock);
 199 }
 200
 201 /*
 202  * caller must hold freezer->lock
 203  */
 204 static void update_freezer_state(struct cgroup *cgroup,
 205                                  struct freezer *freezer)
 206 {
 207         struct cgroup_iter it;
 208         struct task_struct *task;
 209         unsigned int nfrozen = 0, ntotal = 0;
 210
 211         cgroup_iter_start(cgroup, &it);
 212         while ((task = cgroup_iter_next(cgroup, &it))) {
 213                 ntotal++;
 214                 if (is_task_frozen_enough(task))
 215                         nfrozen++;
 216         }
 217
 218         /*
 219          * Transition to FROZEN when no new tasks can be added ensures
 220          * that we never exist in the FROZEN state while there are unfrozen
 221          * tasks.
 222          */
 223         if (nfrozen == ntotal)
 224                 freezer->state = CGROUP_FROZEN;
 225         else if (nfrozen > 0)
 226                 freezer->state = CGROUP_FREEZING;
 227         else
 228                 freezer->state = CGROUP_THAWED;
 229         cgroup_iter_end(cgroup, &it);
 230 }
 231
 232 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
 233                         struct seq_file *m)
 234 {
 235         struct freezer *freezer;
 236         enum freezer_state state;
 237
 238         if (!cgroup_lock_live_group(cgroup))
 239                 return -ENODEV;
 240
 241         freezer = cgroup_freezer(cgroup);
 242         spin_lock_irq(&freezer->lock);
 243         state = freezer->state;
 244         if (state == CGROUP_FREEZING) {
 245                 /* We change from FREEZING to FROZEN lazily if the cgroup was
 246                  * only partially frozen when we exitted write. */
 247                 update_freezer_state(cgroup, freezer);
 248                 state = freezer->state;
 249         }
 250         spin_unlock_irq(&freezer->lock);
 251         cgroup_unlock();
 252
 253         seq_puts(m, freezer_state_strs[state]);
 254         seq_putc(m, '\n');
 255         return 0;
 256 }
 257
 258 static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 259 {
 260         struct cgroup_iter it;
 261         struct task_struct *task;
 262         unsigned int num_cant_freeze_now = 0;
 263
 264         freezer->state = CGROUP_FREEZING;
 265         cgroup_iter_start(cgroup, &it);
 266         while ((task = cgroup_iter_next(cgroup, &it))) {
 267                 if (!freeze_task(task, true))
 268                         continue;
 269                 if (is_task_frozen_enough(task))
 270                         continue;
 271                 if (!freezing(task) && !freezer_should_skip(task))
 272                         num_cant_freeze_now++;
 273         }
 274         cgroup_iter_end(cgroup, &it);
 275
 276         return num_cant_freeze_now ? -EBUSY : 0;
 277 }
 278
 279 static int unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 280 {
 281         struct cgroup_iter it;
 282         struct task_struct *task;
 283
 284         cgroup_iter_start(cgroup, &it);
 285         while ((task = cgroup_iter_next(cgroup, &it))) {
 286                 int do_wake;
 287
 288                 task_lock(task);
 289                 do_wake = __thaw_process(task);
 290                 task_unlock(task);
 291                 if (do_wake)
 292                         wake_up_process(task);
 293         }
 294         cgroup_iter_end(cgroup, &it);
 295         freezer->state = CGROUP_THAWED;
 296
 297         return 0;
 298 }
 299
 300 static int freezer_change_state(struct cgroup *cgroup,
 301                                 enum freezer_state goal_state)
 302 {
 303         struct freezer *freezer;
 304         int retval = 0;
 305
 306         freezer = cgroup_freezer(cgroup);
 307         spin_lock_irq(&freezer->lock);
 308         update_freezer_state(cgroup, freezer);
 309         if (goal_state == freezer->state)
 310                 goto out;
 311         switch (freezer->state) {
 312         case CGROUP_THAWED:
 313                 retval = try_to_freeze_cgroup(cgroup, freezer);
 314                 break;
 315         case CGROUP_FREEZING:
 316                 if (goal_state == CGROUP_FROZEN) {
 317                         /* Userspace is retrying after
 318                          * "/bin/echo FROZEN > freezer.state" returned -EBUSY */
 319                         retval = try_to_freeze_cgroup(cgroup, freezer);
 320                         break;
 321                 }
 322                 /* state == FREEZING and goal_state == THAWED, so unfreeze */
 323         case CGROUP_FROZEN:
 324                 retval = unfreeze_cgroup(cgroup, freezer);
 325                 break;
 326         default:
 327                 break;
 328         }
 329 out:
 330         spin_unlock_irq(&freezer->lock);
 331
 332         return retval;
 333 }
 334
 335 static int freezer_write(struct cgroup *cgroup,
 336                          struct cftype *cft,
 337                          const char *buffer)
 338 {
 339         int retval;
 340         enum freezer_state goal_state;
 341
 342         if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
 343                 goal_state = CGROUP_THAWED;
 344         else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
 345                 goal_state = CGROUP_FROZEN;
 346         else
 347                 return -EIO;
 348
 349         if (!cgroup_lock_live_group(cgroup))
 350                 return -ENODEV;
 351         retval = freezer_change_state(cgroup, goal_state);
 352         cgroup_unlock();
 353         return retval;
 354 }
 355
 356 static struct cftype files[] = {
 357         {
 358                 .name = "state",
 359                 .read_seq_string = freezer_read,
 360                 .write_string = freezer_write,
 361         },
 362 };
 363
 364 static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
 365 {
 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 };