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There are 4 state transitions possible for a freezer.  Only FREEZING ->
FROZEN transaction is done lazily.  This patch allows update_freezer_state
only to perform this transaction and renames the function to
update_if_frozen.

Moreover is_task_frozen_enough function is removed and its every occurence
is replaced with frozen().  Therefore for a group to become FROZEN every
task must be frozen.

The previous version could trigger a following bug: When cgroup is in the
process of freezing (but none of its tasks are frozen yet),
update_freezer_state() (called from freezer_read or freezer_write) would
incorrectly report that a group is 'THAWED' (because nfrozen = 0),
allowing the transaction FREEZING -> THAWED without writing anything to
'freezer.state'.  This is incorrect according to the documentation.  This
could result in a 'THAWED' cgroup with frozen tasks inside.

A code to reproduce this bug is available here:
http://pentium.hopto.o...