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: 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>
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 557f367..0b0e105 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -152,27 +152,38 @@
/*
* 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 @@
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 @@
.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,
