Commit 8cfe400c authored by Tejun Heo's avatar Tejun Heo Committed by Rafael J. Wysocki

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
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: default avatarTejun Heo <>
Signed-off-by: default avatarRafael J. Wysocki <>
parent 133f1128
......@@ -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() 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 {
......@@ -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) &&
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