[RFC][PATCH 3/7] Freezer: Close theoretical race between refrigerator and thaw_tasks

["Rafael J. Wysocki" <rjw@sisk.pl>]

From: Rafael J. Wysocki <rjw@sisk.pl>

If the freezing of tasks fails and a task is preempted in refrigerator() before
calling frozen_process(), then thaw_tasks() may run before this task is frozen.
In that case the task will freeze and no one will thaw it.

To fix this race we can call freezing(current) in refrigerator() along with
frozen_process(current) under the task_lock() which also should be taken in
the error path of try_to_freeze_tasks() as well as in thaw_process().  Moreover,
if thaw_process() additionally clears TIF_FREEZE for tasks that are not frozen,
we can be sure that all tasks are thawed and there are no pending "freeze"
requests after thaw_tasks() has run.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
 include/linux/freezer.h |    4 ++++
 kernel/power/process.c  |   12 +++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

Index: linux-2.6.20-mm2/include/linux/freezer.h
===================================================================
--- linux-2.6.20-mm2.orig/include/linux/freezer.h
+++ linux-2.6.20-mm2/include/linux/freezer.h
@@ -40,11 +40,15 @@ static inline void do_not_freeze(struct 
  */
 static inline int thaw_process(struct task_struct *p)
 {
+	task_lock(p);
 	if (frozen(p)) {
 		p->flags &= ~PF_FROZEN;
+		task_unlock(p);
 		wake_up_process(p);
 		return 1;
 	}
+	clear_tsk_thread_flag(p, TIF_FREEZE);
+	task_unlock(p);
 	return 0;
 }
 
Index: linux-2.6.20-mm2/kernel/power/process.c
===================================================================
--- linux-2.6.20-mm2.orig/kernel/power/process.c
+++ linux-2.6.20-mm2/kernel/power/process.c
@@ -39,10 +39,18 @@ void refrigerator(void)
 	/* Hmm, should we be allowed to suspend when there are realtime
 	   processes around? */
 	long save;
+
+	task_lock(current);
+	if (freezing(current)) {
+		frozen_process(current);
+		task_unlock(current);
+	} else {
+		task_unlock(current);
+		return;
+	}
 	save = current->state;
 	pr_debug("%s entered refrigerator\n", current->comm);
 
-	frozen_process(current);
 	spin_lock_irq(&current->sighand->siglock);
 	recalc_sigpending(); /* We sent fake signal, clean it up */
 	spin_unlock_irq(&current->sighand->siglock);
@@ -159,10 +167,12 @@ static unsigned int try_to_freeze_tasks(
 			if (is_user_space(p) == !freeze_user_space)
 				continue;
 
+			task_lock(p);
 			if (freezeable(p) && !frozen(p))
 				printk(KERN_ERR " %s\n", p->comm);
 
 			cancel_freezing(p);
+			task_unlock(p);
 		} while_each_thread(g, p);
 		read_unlock(&tasklist_lock);
 	}