Commit 8755ade6 authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup_freezer: allow moving tasks in and out of a frozen cgroup 



cgroup_freezer is one of the few users of cgroup_subsys->can_attach()
and uses it to prevent tasks from being migrated into or out of a
frozen cgroup.  This makes cgroup_freezer cumbersome to use especially
when co-mounted with other controllers.

->can_attach() is problematic in general as it can make co-mounting
multiple cgroups difficult - migrating tasks may fail for reasons
completely irrelevant for other controllers.  freezer_can_attach() in
particular is more problematic because it messes with cgroup internal
locking to ensure that the state verification performed at
freezer_can_attach() stays valid until migration is complete.

This patch replaces freezer_can_attach() with freezer_attach() so that
tasks are always allowed to migrate - they are nudged into the
conforming state from freezer_attach().  This means that there can be
tasks which are being migrated which don't conform to the current
cgroup_freezer state until freezer_attach() is complete.  Under the
current locking scheme, the only such place is freezer_fork() which is
updated to handle such window.

While this patch doesn't remove the use of internal cgroup locking
from freezer_read/write() paths, it removes the requirement to keep
the freezer state constant while migrating and enables such change.

Note that this creates a userland visible behavior change - FROZEN
cgroup can no longer be used to lock migrations in and out of the
cgroup.  This behavior change is intended.  I don't think the feature
is necessary - userland should coordinate accesses to cgroup fs anyway
- and even if the feature is needed cgroup_freezer is the completely
wrong place to implement it.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
LKML-Reference: <1350426526-14254-1-git-send-email-tj@kernel.org>
Cc: Matt Helsley <matthltc@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Li Zefan <lizefan@huawei.com>
parent 3c426d5e

kernel/cgroup_freezer.c

+31 −20
Original line number Diff line number Diff line
@@ -152,27 +152,38 @@ static void freezer_destroy(struct cgroup *cgroup) 


/*

 * The call to cgroup_lock() in the freezer.state write method prevents

 * a write to that file racing against an attach, and hence the

 * can_attach() result will remain valid until the attach completes.

 * a write to that file racing against an attach, and hence we don't need

 * to worry about racing against migration.

 */

static int freezer_can_attach(struct cgroup *new_cgroup,

			      struct cgroup_taskset *tset)

static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)

{

	struct freezer *freezer;

	struct freezer *freezer = cgroup_freezer(new_cgrp);

	struct task_struct *task;



	spin_lock_irq(&freezer->lock);



	/*

	 * Anything frozen can't move or be moved to/from.

	 * Make the new tasks conform to the current state of @new_cgrp.

	 * For simplicity, when migrating any task to a FROZEN cgroup, we

	 * revert it to FREEZING and let update_if_frozen() determine the

	 * correct state later.

	 *

	 * Tasks in @tset are on @new_cgrp but may not conform to its

	 * current state before executing the following - !frozen tasks may

	 * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.

	 * This means that, to determine whether to freeze, one should test

	 * whether the state equals THAWED.

	 */

	cgroup_taskset_for_each(task, new_cgroup, tset)

		if (cgroup_freezing(task))

			return -EBUSY;



	freezer = cgroup_freezer(new_cgroup);

	if (freezer->state != CGROUP_THAWED)

		return -EBUSY;

	cgroup_taskset_for_each(task, new_cgrp, tset) {

		if (freezer->state == CGROUP_THAWED) {

			__thaw_task(task);

		} else {

			freeze_task(task);

			freezer->state = CGROUP_FREEZING;

		}

	}



	return 0;

	spin_unlock_irq(&freezer->lock);

}



static void freezer_fork(struct task_struct *task)
@@ -190,12 +201,12 @@ static void freezer_fork(struct task_struct *task) 
		goto out;



	spin_lock_irq(&freezer->lock);

	BUG_ON(freezer->state == CGROUP_FROZEN);



	/* Locking avoids race with FREEZING -> THAWED transitions. */

	if (freezer->state == CGROUP_FREEZING)

	/*

	 * @task might have been just migrated into a FROZEN cgroup.  Test

	 * equality with THAWED.  Read the comment in freezer_attach().

	 */

	if (freezer->state != CGROUP_THAWED)

		freeze_task(task);



	spin_unlock_irq(&freezer->lock);

out:

	rcu_read_unlock();
@@ -352,7 +363,7 @@ struct cgroup_subsys freezer_subsys = { 
	.create		= freezer_create,

	.destroy	= freezer_destroy,

	.subsys_id	= freezer_subsys_id,

	.can_attach	= freezer_can_attach,

	.attach		= freezer_attach,

	.fork		= freezer_fork,

	.base_cftypes	= files,

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