freezer: remove racy clear_freeze_flag() and set PF_NOFREEZE on dead tasks

clear_freeze_flag() in exit_mm() is racy.  Freezing can start
afterwards.  Remove it.  Skipping freezer for exiting task will be
properly implemented later.

Also, freezable() was testing exit_state directly to make system
freezer ignore dead tasks.  Let the exiting task set PF_NOFREEZE after
entering TASK_DEAD instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index d0b7d988f873..95a4141d07e7 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);
         task_unlock(tsk);
         mm_update_next_owner(mm);
         mmput(mm);
@@ -1040,6 +1038,7 @@ NORET_TYPE void do_exit(long code)
         exit_rcu();
         /* causes final put_task_struct in finish_task_switch(). */
         tsk->state = TASK_DEAD;
+        tsk->flags |= PF_NOFREEZE;      /* tell freezer to ignore us */
         schedule();
         BUG();
         /* Avoid "noreturn function does return".  */