freezer: use dedicated lock instead of task_lock() + memory barrier

Freezer synchronization is needlessly complicated - it's by no means a
hot path and the priority is staying unintrusive and safe.  This patch
makes it simply use a dedicated lock instead of piggy-backing on
task_lock() and playing with memory barriers.

On the failure path of try_to_freeze_tasks(), locking is moved from it
to cancel_freezing().  This makes the frozen() test racy but the race
here is a non-issue as the warning is printed for tasks which failed
to enter frozen for 20 seconds and race on PF_FROZEN at the last
moment doesn't change anything.

This simplifies freezer implementation and eases further changes
including some race fixes.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 0 insertions, 2 deletions

diff --git a/kernel/power/process.c b/kernel/power/process.c
index 9db048fb0d70..bd420ca48261 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -118,11 +118,9 @@ static int try_to_freeze_tasks(bool sig_only)
 
                 read_lock(&tasklist_lock);
                 do_each_thread(g, p) {
-                        task_lock(p);
                         if (!wakeup && freezing(p) && !freezer_should_skip(p))
                                 sched_show_task(p);
                         cancel_freezing(p);
-                        task_unlock(p);
                 } while_each_thread(g, p);
                 read_unlock(&tasklist_lock);
         } else {