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>