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freezer: fix current->state restoration race in refrigerator()

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refrigerator() saves current->state before entering frozen state and
restores it before returning using __set_current_state(); however,
this is racy, for example, please consider the following sequence.

	set_current_state(TASK_INTERRUPTIBLE);
	try_to_freeze();
	if (kthread_should_stop())
		break;
	schedule();

If kthread_stop() races with ->state restoration, the restoration can
restore ->state to TASK_INTERRUPTIBLE after kthread_stop() sets it to
TASK_RUNNING but kthread_should_stop() may still see zero
->should_stop because there's no memory barrier between restoring
TASK_INTERRUPTIBLE and kthread_should_stop() test.

This isn't restricted to kthread_should_stop().  current->state is
often used in memory barrier based synchronization and silently
restoring it w/o mb breaks them.

Use set_current_state() instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2011-11-21 12:32:22 -08:00
parent 6fe4c6d466
commit 50fb4f7fc9
1 changed files with 7 additions and 1 deletions
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8
kernel/freezer.c
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@ -58,7 +58,13 @@ void refrigerator(void)
current->flags &= ~PF_FREEZING;
pr_debug("%s left refrigerator\n", current->comm);
__set_current_state(save);
/*
* Restore saved task state before returning. The mb'd version
* needs to be used; otherwise, it might silently break
* synchronization which depends on ordered task state change.
*/
set_current_state(save);
}
EXPORT_SYMBOL(refrigerator);