freezer: close potential race between refrigerator and thaw_tasks

If the freezing of tasks fails and a task is preempted in refrigerator()
before calling frozen_process(), then thaw_tasks() may run before this task is
frozen.  In that case the task will freeze and no one will thaw it.

To fix this race we can call freezing(current) in refrigerator() along with
frozen_process(current) under the task_lock() which also should be taken in
the error path of try_to_freeze_tasks() as well as in thaw_process().
Moreover, if thaw_process() additionally clears TIF_FREEZE for tasks that are
not frozen, we can be sure that all tasks are thawed and there are no pending
"freeze" requests after thaw_tasks() has run.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 1 deletions

diff --git a/kernel/power/process.c b/kernel/power/process.c
index 0884193..02e490e 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -37,10 +37,18 @@ void refrigerator(void)
 	/* Hmm, should we be allowed to suspend when there are realtime
 	   processes around? */
 	long save;
+
+	task_lock(current);
+	if (freezing(current)) {
+		frozen_process(current);
+		task_unlock(current);
+	} else {
+		task_unlock(current);
+		return;
+	}
 	save = current->state;
 	pr_debug("%s entered refrigerator\n", current->comm);
 
-	frozen_process(current);
 	spin_lock_irq(&current->sighand->siglock);
 	recalc_sigpending(); /* We sent fake signal, clean it up */
 	spin_unlock_irq(&current->sighand->siglock);
@@ -152,10 +160,12 @@ static unsigned int try_to_freeze_tasks(int freeze_user_space)
 			if (is_user_space(p) == !freeze_user_space)
 				continue;
 
+			task_lock(p);
 			if (freezeable(p) && !frozen(p))
 				printk(KERN_ERR " %s\n", p->comm);
 
 			cancel_freezing(p);
+			task_unlock(p);
 		} while_each_thread(g, p);
 		read_unlock(&tasklist_lock);
 	}