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/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
  25 enum freezer_state {
  26         CGROUP_THAWED = 0,
  27         CGROUP_FREEZING,
  28         CGROUP_FROZEN,
  29 };
  30
  31 struct freezer {
  32         struct cgroup_subsys_state css;
  33         enum freezer_state state;
  34         spinlock_t lock; /* protects _writes_ to state */
  35 };
  36
  37 static inline struct freezer *cgroup_freezer(
  38                 struct cgroup *cgroup)
  39 {
  40         return container_of(
  41                 cgroup_subsys_state(cgroup, freezer_subsys_id),
  42                 struct freezer, css);
  43 }
  44
  45 static inline struct freezer *task_freezer(struct task_struct *task)
  46 {
  47         return container_of(task_subsys_state(task, freezer_subsys_id),
  48                             struct freezer, css);
  49 }
  50
  51 bool cgroup_freezing(struct task_struct *task)
  52 {
  53         enum freezer_state state;
  54         bool ret;
  55
  56         rcu_read_lock();
  57         state = task_freezer(task)->state;
  58         ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
  59         rcu_read_unlock();
  60
  61         return ret;
  62 }
  63
  64 /*
  65  * cgroups_write_string() limits the size of freezer state strings to
  66  * CGROUP_LOCAL_BUFFER_SIZE
  67  */
  68 static const char *freezer_state_strs[] = {
  69         "THAWED",
  70         "FREEZING",
  71         "FROZEN",
  72 };
  73
  74 /*
  75  * State diagram
  76  * Transitions are caused by userspace writes to the freezer.state file.
  77  * The values in parenthesis are state labels. The rest are edge labels.
  78  *
  79  * (THAWED) --FROZEN--> (FREEZING) --FROZEN--> (FROZEN)
  80  *    ^ ^                    |                     |
  81  *    | \_______THAWED_______/                     |
  82  *    \__________________________THAWED____________/
  83  */
  84
  85 struct cgroup_subsys freezer_subsys;
  86
  87 /* Locks taken and their ordering
  88  * ------------------------------
  89  * cgroup_mutex (AKA cgroup_lock)
  90  * freezer->lock
  91  * css_set_lock
  92  * task->alloc_lock (AKA task_lock)
  93  * task->sighand->siglock
  94  *
  95  * cgroup code forces css_set_lock to be taken before task->alloc_lock
  96  *
  97  * freezer_create(), freezer_destroy():
  98  * cgroup_mutex [ by cgroup core ]
  99  *
 100  * freezer_can_attach():
 101  * cgroup_mutex (held by caller of can_attach)
 102  *
 103  * freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
 104  * freezer->lock
 105  *  sighand->siglock (if the cgroup is freezing)
 106  *
 107  * freezer_read():
 108  * cgroup_mutex
 109  *  freezer->lock
 110  *   write_lock css_set_lock (cgroup iterator start)
 111  *    task->alloc_lock
 112  *   read_lock css_set_lock (cgroup iterator start)
 113  *
 114  * freezer_write() (freeze):
 115  * cgroup_mutex
 116  *  freezer->lock
 117  *   write_lock css_set_lock (cgroup iterator start)
 118  *    task->alloc_lock
 119  *   read_lock css_set_lock (cgroup iterator start)
 120  *    sighand->siglock (fake signal delivery inside freeze_task())
 121  *
 122  * freezer_write() (unfreeze):
 123  * cgroup_mutex
 124  *  freezer->lock
 125  *   write_lock css_set_lock (cgroup iterator start)
 126  *    task->alloc_lock
 127  *   read_lock css_set_lock (cgroup iterator start)
 128  *    task->alloc_lock (inside __thaw_task(), prevents race with refrigerator())
 129  *     sighand->siglock
 130  */
 131 static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
 132                                                   struct cgroup *cgroup)
 133 {
 134         struct freezer *freezer;
 135
 136         freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
 137         if (!freezer)
 138                 return ERR_PTR(-ENOMEM);
 139
 140         spin_lock_init(&freezer->lock);
 141         freezer->state = CGROUP_THAWED;
 142         return &freezer->css;
 143 }
 144
 145 static void freezer_destroy(struct cgroup_subsys *ss,
 146                             struct cgroup *cgroup)
 147 {
 148         struct freezer *freezer = cgroup_freezer(cgroup);
 149
 150         if (freezer->state != CGROUP_THAWED)
 151                 atomic_dec(&system_freezing_cnt);
 152         kfree(freezer);
 153 }
 154
 155 /* task is frozen or will freeze immediately when next it gets woken */
 156 static bool is_task_frozen_enough(struct task_struct *task)
 157 {
 158         return frozen(task) ||
 159                 (task_is_stopped_or_traced(task) && freezing(task));
 160 }
 161
 162 /*
 163  * The call to cgroup_lock() in the freezer.state write method prevents
 164  * a write to that file racing against an attach, and hence the
 165  * can_attach() result will remain valid until the attach completes.
 166  */
 167 static int freezer_can_attach(struct cgroup_subsys *ss,
 168                               struct cgroup *new_cgroup,
 169                               struct cgroup_taskset *tset)
 170 {
 171         struct freezer *freezer;
 172         struct task_struct *task;
 173
 174         /*
 175          * Anything frozen can't move or be moved to/from.
 176          */
 177         cgroup_taskset_for_each(task, new_cgroup, tset)
 178                 if (cgroup_freezing(task))
 179                         return -EBUSY;
 180
 181         freezer = cgroup_freezer(new_cgroup);
 182         if (freezer->state != CGROUP_THAWED)
 183                 return -EBUSY;
 184
 185         return 0;
 186 }
 187
 188 static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
 189 {
 190         struct freezer *freezer;
 191
 192         /*
 193          * No lock is needed, since the task isn't on tasklist yet,
 194          * so it can't be moved to another cgroup, which means the
 195          * freezer won't be removed and will be valid during this
 196          * function call.  Nevertheless, apply RCU read-side critical
 197          * section to suppress RCU lockdep false positives.
 198          */
 199         rcu_read_lock();
 200         freezer = task_freezer(task);
 201         rcu_read_unlock();
 202
 203         /*
 204          * The root cgroup is non-freezable, so we can skip the
 205          * following check.
 206          */
 207         if (!freezer->css.cgroup->parent)
 208                 return;
 209
 210         spin_lock_irq(&freezer->lock);
 211         BUG_ON(freezer->state == CGROUP_FROZEN);
 212
 213         /* Locking avoids race with FREEZING -> THAWED transitions. */
 214         if (freezer->state == CGROUP_FREEZING)
 215                 freeze_task(task);
 216         spin_unlock_irq(&freezer->lock);
 217 }
 218
 219 /*
 220  * caller must hold freezer->lock
 221  */
 222 static void update_if_frozen(struct cgroup *cgroup,
 223                                  struct freezer *freezer)
 224 {
 225         struct cgroup_iter it;
 226         struct task_struct *task;
 227         unsigned int nfrozen = 0, ntotal = 0;
 228         enum freezer_state old_state = freezer->state;
 229
 230         cgroup_iter_start(cgroup, &it);
 231         while ((task = cgroup_iter_next(cgroup, &it))) {
 232                 ntotal++;
 233                 if (freezing(task) && is_task_frozen_enough(task))
 234                         nfrozen++;
 235         }
 236
 237         if (old_state == CGROUP_THAWED) {
 238                 BUG_ON(nfrozen > 0);
 239         } else if (old_state == CGROUP_FREEZING) {
 240                 if (nfrozen == ntotal)
 241                         freezer->state = CGROUP_FROZEN;
 242         } else { /* old_state == CGROUP_FROZEN */
 243                 BUG_ON(nfrozen != ntotal);
 244         }
 245
 246         cgroup_iter_end(cgroup, &it);
 247 }
 248
 249 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
 250                         struct seq_file *m)
 251 {
 252         struct freezer *freezer;
 253         enum freezer_state state;
 254
 255         if (!cgroup_lock_live_group(cgroup))
 256                 return -ENODEV;
 257
 258         freezer = cgroup_freezer(cgroup);
 259         spin_lock_irq(&freezer->lock);
 260         state = freezer->state;
 261         if (state == CGROUP_FREEZING) {
 262                 /* We change from FREEZING to FROZEN lazily if the cgroup was
 263                  * only partially frozen when we exitted write. */
 264                 update_if_frozen(cgroup, freezer);
 265                 state = freezer->state;
 266         }
 267         spin_unlock_irq(&freezer->lock);
 268         cgroup_unlock();
 269
 270         seq_puts(m, freezer_state_strs[state]);
 271         seq_putc(m, '\n');
 272         return 0;
 273 }
 274
 275 static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 276 {
 277         struct cgroup_iter it;
 278         struct task_struct *task;
 279         unsigned int num_cant_freeze_now = 0;
 280
 281         cgroup_iter_start(cgroup, &it);
 282         while ((task = cgroup_iter_next(cgroup, &it))) {
 283                 if (!freeze_task(task))
 284                         continue;
 285                 if (is_task_frozen_enough(task))
 286                         continue;
 287                 if (!freezing(task) && !freezer_should_skip(task))
 288                         num_cant_freeze_now++;
 289         }
 290         cgroup_iter_end(cgroup, &it);
 291
 292         return num_cant_freeze_now ? -EBUSY : 0;
 293 }
 294
 295 static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 296 {
 297         struct cgroup_iter it;
 298         struct task_struct *task;
 299
 300         cgroup_iter_start(cgroup, &it);
 301         while ((task = cgroup_iter_next(cgroup, &it)))
 302                 __thaw_task(task);
 303         cgroup_iter_end(cgroup, &it);
 304 }
 305
 306 static int freezer_change_state(struct cgroup *cgroup,
 307                                 enum freezer_state goal_state)
 308 {
 309         struct freezer *freezer;
 310         int retval = 0;
 311
 312         freezer = cgroup_freezer(cgroup);
 313
 314         spin_lock_irq(&freezer->lock);
 315
 316         update_if_frozen(cgroup, freezer);
 317
 318         switch (goal_state) {
 319         case CGROUP_THAWED:
 320                 if (freezer->state != CGROUP_THAWED)
 321                         atomic_dec(&system_freezing_cnt);
 322                 freezer->state = CGROUP_THAWED;
 323                 unfreeze_cgroup(cgroup, freezer);
 324                 break;
 325         case CGROUP_FROZEN:
 326                 if (freezer->state == CGROUP_THAWED)
 327                         atomic_inc(&system_freezing_cnt);
 328                 freezer->state = CGROUP_FREEZING;
 329                 retval = try_to_freeze_cgroup(cgroup, freezer);
 330                 break;
 331         default:
 332                 BUG();
 333         }
 334
 335         spin_unlock_irq(&freezer->lock);
 336
 337         return retval;
 338 }
 339
 340 static int freezer_write(struct cgroup *cgroup,
 341                          struct cftype *cft,
 342                          const char *buffer)
 343 {
 344         int retval;
 345         enum freezer_state goal_state;
 346
 347         if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
 348                 goal_state = CGROUP_THAWED;
 349         else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
 350                 goal_state = CGROUP_FROZEN;
 351         else
 352                 return -EINVAL;
 353
 354         if (!cgroup_lock_live_group(cgroup))
 355                 return -ENODEV;
 356         retval = freezer_change_state(cgroup, goal_state);
 357         cgroup_unlock();
 358         return retval;
 359 }
 360
 361 static struct cftype files[] = {
 362         {
 363                 .name = "state",
 364                 .read_seq_string = freezer_read,
 365                 .write_string = freezer_write,
 366         },
 367 };
 368
 369 static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
 370 {
 371         if (!cgroup->parent)
 372                 return 0;
 373         return cgroup_add_files(cgroup, ss, files, ARRAY_SIZE(files));
 374 }
 375
 376 struct cgroup_subsys freezer_subsys = {
 377         .name           = "freezer",
 378         .create         = freezer_create,
 379         .destroy        = freezer_destroy,
 380         .populate       = freezer_populate,
 381         .subsys_id      = freezer_subsys_id,
 382         .can_attach     = freezer_can_attach,
 383         .fork           = freezer_fork,
 384 };