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>

1 files changed, 6 insertions, 0 deletions

diff --git a/kernel/power/process.c b/kernel/power/process.c
index e50b4c1b2a0f..eb2c88a9e562 100644
--- a/kernel/power/process.c
+++ b/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))