[PATCH 06/16] freezer: make exiting tasks properly unfreezable

[Tejun Heo <tj@kernel.org>]

There's no point in freezing an exiting task.  The current code
seemingly tries that by calling clear_freeze_flag() from exit_mm() but
it's racy as freeze might happen afterwards.

This patch removes the racy clear_freeze_flag() makes do_exit() set
PF_NOFREEZE after PTRACE_EVENT_EXIT, after which freezing doesn't make
sense.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/exit.c          |    8 ++++++--
 kernel/power/process.c |    3 +--
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 2913b35..ac58259 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -679,8 +679,6 @@ static void exit_mm(struct task_struct * tsk)
 	tsk->mm = NULL;
 	up_read(&mm->mmap_sem);
 	enter_lazy_tlb(mm, current);
-	/* We don't want this task to be frozen prematurely */
-	clear_freeze_flag(tsk);
 	if (tsk->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
 		atomic_dec(&mm->oom_disable_count);
 	task_unlock(tsk);
@@ -915,6 +913,12 @@ NORET_TYPE void do_exit(long code)
 
 	ptrace_event(PTRACE_EVENT_EXIT, code);
 
+	/*
+	 * With ptrace notification done, there's no point in freezing from
+	 * here on.  Disallow freezing.
+	 */
+	current->flags |= PF_NOFREEZE;
+
 	validate_creds_for_do_exit(tsk);
 
 	/*
diff --git a/kernel/power/process.c b/kernel/power/process.c
index bec09c3..ddfaba4 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -25,8 +25,7 @@
 static inline int freezable(struct task_struct * p)
 {
 	if ((p == current) ||
-	    (p->flags & PF_NOFREEZE) ||
-	    (p->exit_state != 0))
+	    (p->flags & PF_NOFREEZE))
 		return 0;
 	return 1;
 }
-- 
1.7.6