freezer: add missing mb's to freezer_count() and freezer_should_skip()

freezer: add missing mb's to freezer_count() and freezer_should_skip()

commit dd67d32dbc5de299d70cc9e10c6c1e29ffa56b92 upstream.

A task is considered frozen enough between freezer_do_not_count() and
freezer_count() and freezers use freezer_should_skip() to test this
condition.  This supposedly works because freezer_count() always calls
try_to_freezer() after clearing %PF_FREEZER_SKIP.

However, there currently is nothing which guarantees that
freezer_count() sees %true freezing() after clearing %PF_FREEZER_SKIP
when freezing is in progress, and vice-versa.  A task can escape the
freezing condition in effect by freezer_count() seeing !freezing() and
freezer_should_skip() seeing %PF_FREEZER_SKIP.

This patch adds smp_mb()'s to freezer_count() and
freezer_should_skip() such that either %true freezing() is visible to
freezer_count() or !PF_FREEZER_SKIP is visible to
freezer_should_skip().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
[bwh: Backported to 3.2:
 - Adjust context and indentation
 - freezer_do_not_count() and freezer_count() are no-ops for kernel tasks]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>