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>
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@ -104,8 +104,13 @@ bool freeze_task(struct task_struct *p, bool sig_only)
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}
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}
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if (should_send_signal(p)) {
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if (should_send_signal(p)) {
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if (!signal_pending(p))
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fake_signal_wake_up(p);
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fake_signal_wake_up(p);
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/*
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* fake_signal_wake_up() goes through p's scheduler
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* lock and guarantees that TASK_STOPPED/TRACED ->
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* TASK_RUNNING transition can't race with task state
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* testing in try_to_freeze_tasks().
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*/
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} else if (sig_only) {
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} else if (sig_only) {
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return false;
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return false;
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} else {
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} else {
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@ -64,6 +64,12 @@ static int try_to_freeze_tasks(bool sig_only)
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* perturb a task in TASK_STOPPED or TASK_TRACED.
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* perturb a task in TASK_STOPPED or TASK_TRACED.
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* It is "frozen enough". If the task does wake
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* It is "frozen enough". If the task does wake
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* up, it will immediately call try_to_freeze.
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* up, it will immediately call try_to_freeze.
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*
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* Because freeze_task() goes through p's
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* scheduler lock after setting TIF_FREEZE, it's
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* guaranteed that either we see TASK_RUNNING or
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* try_to_stop() after schedule() in ptrace/signal
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* stop sees TIF_FREEZE.
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*/
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*/
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if (!task_is_stopped_or_traced(p) &&
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if (!task_is_stopped_or_traced(p) &&
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!freezer_should_skip(p))
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!freezer_should_skip(p))
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