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>
diff --git a/kernel/freezer.c b/kernel/freezer.c
index bd1d42b..66ecd2e 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -104,8 +104,13 @@
}
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 {
diff --git a/kernel/power/process.c b/kernel/power/process.c
index e50b4c1..eb2c88a 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -64,6 +64,12 @@
* 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))