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 *cgroup)
 132 {
 133         struct freezer *freezer;
 134
 135         freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
 136         if (!freezer)
 137                 return ERR_PTR(-ENOMEM);
 138
 139         spin_lock_init(&freezer->lock);
 140         freezer->state = CGROUP_THAWED;
 141         return &freezer->css;
 142 }
 143
 144 static void freezer_destroy(struct cgroup *cgroup)
 145 {
 146         struct freezer *freezer = cgroup_freezer(cgroup);
 147
 148         if (freezer->state != CGROUP_THAWED)
 149                 atomic_dec(&system_freezing_cnt);
 150         kfree(freezer);
 151 }
 152
 153 /* task is frozen or will freeze immediately when next it gets woken */
 154 static bool is_task_frozen_enough(struct task_struct *task)
 155 {
 156         return frozen(task) ||
 157                 (task_is_stopped_or_traced(task) && freezing(task));
 158 }
 159
 160 /*
 161  * The call to cgroup_lock() in the freezer.state write method prevents
 162  * a write to that file racing against an attach, and hence the
 163  * can_attach() result will remain valid until the attach completes.
 164  */
 165 static int freezer_can_attach(struct cgroup *new_cgroup,
 166                               struct cgroup_taskset *tset)
 167 {
 168         struct freezer *freezer;
 169         struct task_struct *task;
 170
 171         /*
 172          * Anything frozen can't move or be moved to/from.
 173          */
 174         cgroup_taskset_for_each(task, new_cgroup, tset)
 175                 if (cgroup_freezing(task))
 176                         return -EBUSY;
 177
 178         freezer = cgroup_freezer(new_cgroup);
 179         if (freezer->state != CGROUP_THAWED)
 180                 return -EBUSY;
 181
 182         return 0;
 183 }
 184
 185 static void freezer_fork(struct task_struct *task)
 186 {
 187         struct freezer *freezer;
 188
 189         /*
 190          * No lock is needed, since the task isn't on tasklist yet,
 191          * so it can't be moved to another cgroup, which means the
 192          * freezer won't be removed and will be valid during this
 193          * function call.  Nevertheless, apply RCU read-side critical
 194          * section to suppress RCU lockdep false positives.
 195          */
 196         rcu_read_lock();
 197         freezer = task_freezer(task);
 198         rcu_read_unlock();
 199
 200         /*
 201          * The root cgroup is non-freezable, so we can skip the
 202          * following check.
 203          */
 204         if (!freezer->css.cgroup->parent)
 205                 return;
 206
 207         spin_lock_irq(&freezer->lock);
 208         BUG_ON(freezer->state == CGROUP_FROZEN);
 209
 210         /* Locking avoids race with FREEZING -> THAWED transitions. */
 211         if (freezer->state == CGROUP_FREEZING)
 212                 freeze_task(task);
 213         spin_unlock_irq(&freezer->lock);
 214 }
 215
 216 /*
 217  * caller must hold freezer->lock
 218  */
 219 static void update_if_frozen(struct cgroup *cgroup,
 220                                  struct freezer *freezer)
 221 {
 222         struct cgroup_iter it;
 223         struct task_struct *task;
 224         unsigned int nfrozen = 0, ntotal = 0;
 225         enum freezer_state old_state = freezer->state;
 226
 227         cgroup_iter_start(cgroup, &it);
 228         while ((task = cgroup_iter_next(cgroup, &it))) {
 229                 ntotal++;
 230                 if (freezing(task) && is_task_frozen_enough(task))
 231                         nfrozen++;
 232         }
 233
 234         if (old_state == CGROUP_THAWED) {
 235                 BUG_ON(nfrozen > 0);
 236         } else if (old_state == CGROUP_FREEZING) {
 237                 if (nfrozen == ntotal)
 238                         freezer->state = CGROUP_FROZEN;
 239         } else { /* old_state == CGROUP_FROZEN */
 240                 BUG_ON(nfrozen != ntotal);
 241         }
 242
 243         cgroup_iter_end(cgroup, &it);
 244 }
 245
 246 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
 247                         struct seq_file *m)
 248 {
 249         struct freezer *freezer;
 250         enum freezer_state state;
 251
 252         if (!cgroup_lock_live_group(cgroup))
 253                 return -ENODEV;
 254
 255         freezer = cgroup_freezer(cgroup);
 256         spin_lock_irq(&freezer->lock);
 257         state = freezer->state;
 258         if (state == CGROUP_FREEZING) {
 259                 /* We change from FREEZING to FROZEN lazily if the cgroup was
 260                  * only partially frozen when we exitted write. */
 261                 update_if_frozen(cgroup, freezer);
 262                 state = freezer->state;
 263         }
 264         spin_unlock_irq(&freezer->lock);
 265         cgroup_unlock();
 266
 267         seq_puts(m, freezer_state_strs[state]);
 268         seq_putc(m, '\n');
 269         return 0;
 270 }
 271
 272 static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 273 {
 274         struct cgroup_iter it;
 275         struct task_struct *task;
 276         unsigned int num_cant_freeze_now = 0;
 277
 278         cgroup_iter_start(cgroup, &it);
 279         while ((task = cgroup_iter_next(cgroup, &it))) {
 280                 if (!freeze_task(task))
 281                         continue;
 282                 if (is_task_frozen_enough(task))
 283                         continue;
 284                 if (!freezing(task) && !freezer_should_skip(task))
 285                         num_cant_freeze_now++;
 286         }
 287         cgroup_iter_end(cgroup, &it);
 288
 289         return num_cant_freeze_now ? -EBUSY : 0;
 290 }
 291
 292 static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
 293 {
 294         struct cgroup_iter it;
 295         struct task_struct *task;
 296
 297         cgroup_iter_start(cgroup, &it);
 298         while ((task = cgroup_iter_next(cgroup, &it)))
 299                 __thaw_task(task);
 300         cgroup_iter_end(cgroup, &it);
 301 }
 302
 303 static int freezer_change_state(struct cgroup *cgroup,
 304                                 enum freezer_state goal_state)
 305 {
 306         struct freezer *freezer;
 307         int retval = 0;
 308
 309         freezer = cgroup_freezer(cgroup);
 310
 311         spin_lock_irq(&freezer->lock);
 312
 313         update_if_frozen(cgroup, freezer);
 314
 315         switch (goal_state) {
 316         case CGROUP_THAWED:
 317                 if (freezer->state != CGROUP_THAWED)
 318                         atomic_dec(&system_freezing_cnt);
 319                 freezer->state = CGROUP_THAWED;
 320                 unfreeze_cgroup(cgroup, freezer);
 321                 break;
 322         case CGROUP_FROZEN:
 323                 if (freezer->state == CGROUP_THAWED)
 324                         atomic_inc(&system_freezing_cnt);
 325                 freezer->state = CGROUP_FREEZING;
 326                 retval = try_to_freeze_cgroup(cgroup, freezer);
 327                 break;
 328         default:
 329                 BUG();
 330         }
 331
 332         spin_unlock_irq(&freezer->lock);
 333
 334         return retval;
 335 }
 336
 337 static int freezer_write(struct cgroup *cgroup,
 338                          struct cftype *cft,
 339                          const char *buffer)
 340 {
 341         int retval;
 342         enum freezer_state goal_state;
 343
 344         if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
 345                 goal_state = CGROUP_THAWED;
 346         else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
 347                 goal_state = CGROUP_FROZEN;
 348         else
 349                 return -EINVAL;
 350
 351         if (!cgroup_lock_live_group(cgroup))
 352                 return -ENODEV;
 353         retval = freezer_change_state(cgroup, goal_state);
 354         cgroup_unlock();
 355         return retval;
 356 }
 357
 358 static struct cftype files[] = {
 359         {
 360                 .name = "state",
 361                 .flags = CFTYPE_NOT_ON_ROOT,
 362                 .read_seq_string = freezer_read,
 363                 .write_string = freezer_write,
 364         },
 365         { }     /* terminate */
 366 };
 367
 368 struct cgroup_subsys freezer_subsys = {
 369         .name           = "freezer",
 370         .create         = freezer_create,
 371         .destroy        = freezer_destroy,
 372         .subsys_id      = freezer_subsys_id,
 373         .can_attach     = freezer_can_attach,
 374         .fork           = freezer_fork,
 375         .base_cftypes   = files,
 376 };