freezer: fix set_freezable[_with_signal]() race

A kthread doing set_freezable*() may race with on-going PM freeze and
the freezer might think all tasks are frozen while the new freezable
kthread is merrily proceeding to execute code paths which aren't
supposed to be executing during PM freeze.

Reimplement set_freezable[_with_signal]() using __set_freezable() such
that freezable PF flags are modified under freezer_lock and
try_to_freeze() is called afterwards.  This eliminates race condition
against freezing.

Note: Separated out from larger patch to resolve fix order dependency
      Oleg pointed out.

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

1 files changed, 25 insertions, 0 deletions

diff --git a/kernel/freezer.c b/kernel/freezer.c
index 95a1238..b1e7a7b 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -171,3 +171,28 @@ void __thaw_task(struct task_struct *p)
 	}
 	spin_unlock_irqrestore(&freezer_lock, flags);
 }
+
+/**
+ * __set_freezable - make %current freezable
+ * @with_signal: do we want %TIF_SIGPENDING for notification too?
+ *
+ * Mark %current freezable and enter refrigerator if necessary.
+ */
+bool __set_freezable(bool with_signal)
+{
+	might_sleep();
+
+	/*
+	 * Modify flags while holding freezer_lock.  This ensures the
+	 * freezer notices that we aren't frozen yet or the freezing
+	 * condition is visible to try_to_freeze() below.
+	 */
+	spin_lock_irq(&freezer_lock);
+	current->flags &= ~PF_NOFREEZE;
+	if (with_signal)
+		current->flags &= ~PF_FREEZER_NOSIG;
+	spin_unlock_irq(&freezer_lock);
+
+	return try_to_freeze();
+}
+EXPORT_SYMBOL(__set_freezable);