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Commit 8cfe400c authored by Tejun Heo's avatar Tejun Heo Committed by Rafael J. Wysocki
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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: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
parent 133f1128

kernel/freezer.c

+7 −2
Original line number Diff line number Diff line
@@ -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() 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

+6 −0
Original line number Diff line number Diff line
@@ -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))