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

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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: Tejun 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>
This commit is contained in:
Tejun Heo 2012-10-16 15:03:14 -07:00
parent 3c426d5e11
commit 8755ade683
1 changed files with 31 additions and 20 deletions
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51
kernel/cgroup_freezer.c
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@ -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;
cgroup_taskset_for_each(task, new_cgrp, tset) {
if (freezer->state == CGROUP_THAWED) {
__thaw_task(task);
} else {
freeze_task(task);
freezer->state = CGROUP_FREEZING;
}
}
freezer = cgroup_freezer(new_cgroup);
if (freezer->state != CGROUP_THAWED)
return -EBUSY;
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,