![Tejun Heo](/assets/img/avatar_default.png)
cgroup core has a bug which violates a basic rule about event notifications - when a new entity needs to be added, you add that to the notification list first and then make the new entity conform to the current state. If done in the reverse order, an event happening inbetween will be lost. cgroup_subsys->fork() is invoked way before the new task is added to the css_set. Currently, cgroup_freezer is the only user of ->fork() and uses it to make new tasks conform to the current state of the freezer. If FROZEN state is requested while fork is in progress between cgroup_fork_callbacks() and cgroup_post_fork(), the child could escape freezing - the cgroup isn't frozen when ->fork() is called and the freezer couldn't see the new task on the css_set. This patch moves cgroup_subsys->fork() invocation to cgroup_post_fork() after the new task is added to the css_set. cgroup_fork_callbacks() is removed. Because now a task may be migrated during cgroup_subsys->fork(), freezer_fork() is updated so that it adheres to the usual RCU locking and the rather pointless comment on why locking can be different there is removed (if it doesn't make anything simpler, why even bother?). Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Rafael J. Wysocki <rjw@sisk.pl> Cc: stable@vger.kernel.org
380 lines
9.1 KiB
C
380 lines
9.1 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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bool cgroup_freezing(struct task_struct *task)
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{
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enum freezer_state state;
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bool ret;
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rcu_read_lock();
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state = task_freezer(task)->state;
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ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
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rcu_read_unlock();
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return ret;
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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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* 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 *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 *cgroup)
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{
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struct freezer *freezer = cgroup_freezer(cgroup);
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if (freezer->state != CGROUP_THAWED)
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atomic_dec(&system_freezing_cnt);
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kfree(freezer);
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}
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/* task is frozen or will freeze immediately when next it gets woken */
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static bool is_task_frozen_enough(struct task_struct *task)
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{
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return frozen(task) ||
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(task_is_stopped_or_traced(task) && freezing(task));
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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 *new_cgroup,
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struct cgroup_taskset *tset)
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{
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struct freezer *freezer;
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struct task_struct *task;
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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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cgroup_taskset_for_each(task, new_cgroup, tset)
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if (cgroup_freezing(task))
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return -EBUSY;
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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 void freezer_fork(struct task_struct *task)
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{
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struct freezer *freezer;
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rcu_read_lock();
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freezer = task_freezer(task);
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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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goto out;
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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);
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spin_unlock_irq(&freezer->lock);
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out:
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rcu_read_unlock();
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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) && is_task_frozen_enough(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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cgroup_iter_start(cgroup, &it);
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while ((task = cgroup_iter_next(cgroup, &it))) {
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if (!freeze_task(task))
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continue;
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if (is_task_frozen_enough(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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}
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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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switch (goal_state) {
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case CGROUP_THAWED:
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if (freezer->state != CGROUP_THAWED)
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atomic_dec(&system_freezing_cnt);
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freezer->state = 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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if (freezer->state == CGROUP_THAWED)
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atomic_inc(&system_freezing_cnt);
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freezer->state = CGROUP_FREEZING;
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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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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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.flags = CFTYPE_NOT_ON_ROOT,
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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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{ } /* terminate */
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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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.subsys_id = freezer_subsys_id,
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.can_attach = freezer_can_attach,
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.fork = freezer_fork,
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.base_cftypes = files,
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/*
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* freezer subsys doesn't handle hierarchy at all. Frozen state
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* should be inherited through the hierarchy - if a parent is
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* frozen, all its children should be frozen. Fix it and remove
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* the following.
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*/
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.broken_hierarchy = true,
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};
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