Freezer: Fix a race during freezing of TASK_STOPPED tasks

After calling freeze_task(), try_to_freeze_tasks() see whether the
task is stopped or traced and if so, considers it to be frozen;
however, nothing guarantees that either the task being frozen sees
TIF_FREEZE or the freezer sees TASK_STOPPED -> TASK_RUNNING
transition.  The task being frozen may wake up and not see TIF_FREEZE
while the freezer fails to notice the transition and believes the task
is still stopped.

This patch fixes the race by making freeze_task() always go through
fake_signal_wake_up() for applicable tasks.  The function goes through
the target task's scheduler lock and thus guarantees that either the
target sees TIF_FREEZE or try_to_freeze_task() sees TASK_RUNNING.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

kernel/freezer.c

@@ -104,8 +104,13 @@ bool freeze_task(struct task_struct *p, bool sig_only)
 	}
 
 	if (should_send_signal(p)) {
-		if (!signal_pending(p))
-			fake_signal_wake_up(p);
+		fake_signal_wake_up(p);
+		/*
+		 * fake_signal_wake_up() goes through p's scheduler
+		 * lock and guarantees that TASK_STOPPED/TRACED ->
+		 * TASK_RUNNING transition can't race with task state
+		 * testing in try_to_freeze_tasks().
+		 */
 	} else if (sig_only) {
 		return false;
 	} else {

kernel/power/process.c

@@ -64,6 +64,12 @@ static int try_to_freeze_tasks(bool sig_only)
 			 * perturb a task in TASK_STOPPED or TASK_TRACED.
 			 * It is "frozen enough".  If the task does wake
 			 * up, it will immediately call try_to_freeze.
+			 *
+			 * Because freeze_task() goes through p's
+			 * scheduler lock after setting TIF_FREEZE, it's
+			 * guaranteed that either we see TASK_RUNNING or
+			 * try_to_stop() after schedule() in ptrace/signal
+			 * stop sees TIF_FREEZE.
 			 */
 			if (!task_is_stopped_or_traced(p) &&
 			    !freezer_should_skip(p))