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![Tejun Heo](/assets/img/avatar_default.png)
Currently freezing (TIF_FREEZE) and frozen (PF_FROZEN) states are interlocked - freezing is set to request freeze and when the task actually freezes, it clears freezing and sets frozen. This interlocking makes things more complex than necessary - freezing doesn't mean there's freezing condition in effect and frozen doesn't match the task actually entering and leaving frozen state (it's cleared by the thawing task). This patch makes freezing indicate that freeze condition is in effect. A task enters and stays frozen if freezing. This makes PF_FROZEN manipulation done only by the task itself and prevents wakeup from __thaw_task() leaking outside of refrigerator. The only place which needs to tell freezing && !frozen is try_to_freeze_task() to whine about tasks which don't enter frozen. It's updated to test the condition explicitly. With the change, frozen() state my linger after __thaw_task() until the task wakes up and exits fridge. This can trigger BUG_ON() in update_if_frozen(). Work it around by testing freezing() && frozen() instead of frozen(). -v2: Oleg pointed out missing re-check of freezing() when trying to clear FROZEN and possible spurious BUG_ON() trigger in update_if_frozen(). Both fixed. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Paul Menage <paul@paulmenage.org>
389 lines
9.4 KiB
C
389 lines
9.4 KiB
C
/*
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* cgroup_freezer.c - control group freezer subsystem
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*
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* Copyright IBM Corporation, 2007
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*
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* Author : Cedric Le Goater <clg@fr.ibm.com>
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*
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* This program is free software; you can redistribute it and/or modify it
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* under the terms of version 2.1 of the GNU Lesser General Public License
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* as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it would be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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*/
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#include <linux/export.h>
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#include <linux/slab.h>
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#include <linux/cgroup.h>
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#include <linux/fs.h>
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#include <linux/uaccess.h>
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#include <linux/freezer.h>
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#include <linux/seq_file.h>
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enum freezer_state {
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CGROUP_THAWED = 0,
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CGROUP_FREEZING,
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CGROUP_FROZEN,
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};
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struct freezer {
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struct cgroup_subsys_state css;
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enum freezer_state state;
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spinlock_t lock; /* protects _writes_ to state */
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};
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static inline struct freezer *cgroup_freezer(
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struct cgroup *cgroup)
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{
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return container_of(
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cgroup_subsys_state(cgroup, freezer_subsys_id),
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struct freezer, css);
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}
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static inline struct freezer *task_freezer(struct task_struct *task)
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{
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return container_of(task_subsys_state(task, freezer_subsys_id),
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struct freezer, css);
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}
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static inline int __cgroup_freezing_or_frozen(struct task_struct *task)
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{
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enum freezer_state state = task_freezer(task)->state;
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return (state == CGROUP_FREEZING) || (state == CGROUP_FROZEN);
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}
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int cgroup_freezing_or_frozen(struct task_struct *task)
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{
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int result;
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task_lock(task);
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result = __cgroup_freezing_or_frozen(task);
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task_unlock(task);
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return result;
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}
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/*
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* cgroups_write_string() limits the size of freezer state strings to
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* CGROUP_LOCAL_BUFFER_SIZE
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*/
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static const char *freezer_state_strs[] = {
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"THAWED",
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"FREEZING",
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"FROZEN",
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};
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/*
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* State diagram
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* Transitions are caused by userspace writes to the freezer.state file.
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* The values in parenthesis are state labels. The rest are edge labels.
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*
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* (THAWED) --FROZEN--> (FREEZING) --FROZEN--> (FROZEN)
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* ^ ^ | |
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* | \_______THAWED_______/ |
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* \__________________________THAWED____________/
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*/
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struct cgroup_subsys freezer_subsys;
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/* Locks taken and their ordering
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* ------------------------------
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* cgroup_mutex (AKA cgroup_lock)
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* freezer->lock
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* css_set_lock
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* task->alloc_lock (AKA task_lock)
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* task->sighand->siglock
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*
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* cgroup code forces css_set_lock to be taken before task->alloc_lock
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*
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* freezer_create(), freezer_destroy():
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* cgroup_mutex [ by cgroup core ]
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*
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* freezer_can_attach():
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* cgroup_mutex (held by caller of can_attach)
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*
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* cgroup_freezing_or_frozen():
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* task->alloc_lock (to get task's cgroup)
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*
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* freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
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* freezer->lock
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* sighand->siglock (if the cgroup is freezing)
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*
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* freezer_read():
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* cgroup_mutex
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* freezer->lock
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* write_lock css_set_lock (cgroup iterator start)
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* task->alloc_lock
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* read_lock css_set_lock (cgroup iterator start)
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*
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* freezer_write() (freeze):
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* cgroup_mutex
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* freezer->lock
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* write_lock css_set_lock (cgroup iterator start)
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* task->alloc_lock
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* read_lock css_set_lock (cgroup iterator start)
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* sighand->siglock (fake signal delivery inside freeze_task())
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*
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* freezer_write() (unfreeze):
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* cgroup_mutex
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* freezer->lock
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* write_lock css_set_lock (cgroup iterator start)
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* task->alloc_lock
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* read_lock css_set_lock (cgroup iterator start)
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* task->alloc_lock (inside __thaw_task(), prevents race with refrigerator())
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* sighand->siglock
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*/
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static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
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struct cgroup *cgroup)
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{
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struct freezer *freezer;
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freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
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if (!freezer)
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return ERR_PTR(-ENOMEM);
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spin_lock_init(&freezer->lock);
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freezer->state = CGROUP_THAWED;
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return &freezer->css;
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}
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static void freezer_destroy(struct cgroup_subsys *ss,
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struct cgroup *cgroup)
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{
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kfree(cgroup_freezer(cgroup));
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}
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/*
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* The call to cgroup_lock() in the freezer.state write method prevents
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* a write to that file racing against an attach, and hence the
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* can_attach() result will remain valid until the attach completes.
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*/
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static int freezer_can_attach(struct cgroup_subsys *ss,
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struct cgroup *new_cgroup,
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struct task_struct *task)
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{
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struct freezer *freezer;
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/*
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* Anything frozen can't move or be moved to/from.
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*/
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freezer = cgroup_freezer(new_cgroup);
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if (freezer->state != CGROUP_THAWED)
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return -EBUSY;
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return 0;
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}
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static int freezer_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
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{
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rcu_read_lock();
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if (__cgroup_freezing_or_frozen(tsk)) {
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rcu_read_unlock();
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return -EBUSY;
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}
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rcu_read_unlock();
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return 0;
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}
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static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
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{
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struct freezer *freezer;
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/*
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* No lock is needed, since the task isn't on tasklist yet,
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* so it can't be moved to another cgroup, which means the
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* freezer won't be removed and will be valid during this
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* function call. Nevertheless, apply RCU read-side critical
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* section to suppress RCU lockdep false positives.
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*/
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rcu_read_lock();
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freezer = task_freezer(task);
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rcu_read_unlock();
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/*
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* The root cgroup is non-freezable, so we can skip the
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* following check.
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*/
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if (!freezer->css.cgroup->parent)
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return;
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spin_lock_irq(&freezer->lock);
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BUG_ON(freezer->state == CGROUP_FROZEN);
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/* Locking avoids race with FREEZING -> THAWED transitions. */
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if (freezer->state == CGROUP_FREEZING)
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freeze_task(task, true);
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spin_unlock_irq(&freezer->lock);
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}
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/*
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* caller must hold freezer->lock
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*/
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static void update_if_frozen(struct cgroup *cgroup,
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struct freezer *freezer)
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{
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struct cgroup_iter it;
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struct task_struct *task;
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unsigned int nfrozen = 0, ntotal = 0;
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enum freezer_state old_state = freezer->state;
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cgroup_iter_start(cgroup, &it);
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while ((task = cgroup_iter_next(cgroup, &it))) {
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ntotal++;
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if (freezing(task) && frozen(task))
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nfrozen++;
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}
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if (old_state == CGROUP_THAWED) {
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BUG_ON(nfrozen > 0);
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} else if (old_state == CGROUP_FREEZING) {
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if (nfrozen == ntotal)
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freezer->state = CGROUP_FROZEN;
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} else { /* old_state == CGROUP_FROZEN */
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BUG_ON(nfrozen != ntotal);
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}
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cgroup_iter_end(cgroup, &it);
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}
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static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
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struct seq_file *m)
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{
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struct freezer *freezer;
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enum freezer_state state;
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if (!cgroup_lock_live_group(cgroup))
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return -ENODEV;
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freezer = cgroup_freezer(cgroup);
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spin_lock_irq(&freezer->lock);
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state = freezer->state;
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if (state == CGROUP_FREEZING) {
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/* We change from FREEZING to FROZEN lazily if the cgroup was
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* only partially frozen when we exitted write. */
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update_if_frozen(cgroup, freezer);
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state = freezer->state;
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}
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spin_unlock_irq(&freezer->lock);
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cgroup_unlock();
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seq_puts(m, freezer_state_strs[state]);
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seq_putc(m, '\n');
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return 0;
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}
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static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
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{
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struct cgroup_iter it;
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struct task_struct *task;
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unsigned int num_cant_freeze_now = 0;
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freezer->state = CGROUP_FREEZING;
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cgroup_iter_start(cgroup, &it);
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while ((task = cgroup_iter_next(cgroup, &it))) {
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if (!freeze_task(task, true))
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continue;
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if (frozen(task))
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continue;
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if (!freezing(task) && !freezer_should_skip(task))
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num_cant_freeze_now++;
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}
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cgroup_iter_end(cgroup, &it);
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return num_cant_freeze_now ? -EBUSY : 0;
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}
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static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
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{
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struct cgroup_iter it;
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struct task_struct *task;
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cgroup_iter_start(cgroup, &it);
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while ((task = cgroup_iter_next(cgroup, &it)))
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__thaw_task(task);
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cgroup_iter_end(cgroup, &it);
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freezer->state = CGROUP_THAWED;
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}
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static int freezer_change_state(struct cgroup *cgroup,
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enum freezer_state goal_state)
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{
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struct freezer *freezer;
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int retval = 0;
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freezer = cgroup_freezer(cgroup);
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spin_lock_irq(&freezer->lock);
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update_if_frozen(cgroup, freezer);
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if (goal_state == freezer->state)
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goto out;
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switch (goal_state) {
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case CGROUP_THAWED:
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unfreeze_cgroup(cgroup, freezer);
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break;
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case CGROUP_FROZEN:
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retval = try_to_freeze_cgroup(cgroup, freezer);
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break;
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default:
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BUG();
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}
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out:
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spin_unlock_irq(&freezer->lock);
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return retval;
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}
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static int freezer_write(struct cgroup *cgroup,
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struct cftype *cft,
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const char *buffer)
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{
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int retval;
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enum freezer_state goal_state;
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if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
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goal_state = CGROUP_THAWED;
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else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
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goal_state = CGROUP_FROZEN;
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else
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return -EINVAL;
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if (!cgroup_lock_live_group(cgroup))
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return -ENODEV;
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retval = freezer_change_state(cgroup, goal_state);
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cgroup_unlock();
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return retval;
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}
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static struct cftype files[] = {
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{
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.name = "state",
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.read_seq_string = freezer_read,
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.write_string = freezer_write,
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},
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};
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static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
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{
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if (!cgroup->parent)
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return 0;
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return cgroup_add_files(cgroup, ss, files, ARRAY_SIZE(files));
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}
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struct cgroup_subsys freezer_subsys = {
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.name = "freezer",
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.create = freezer_create,
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.destroy = freezer_destroy,
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.populate = freezer_populate,
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.subsys_id = freezer_subsys_id,
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.can_attach = freezer_can_attach,
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.can_attach_task = freezer_can_attach_task,
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.pre_attach = NULL,
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.attach_task = NULL,
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.attach = NULL,
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.fork = freezer_fork,
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.exit = NULL,
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};
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