From: Tejun Heo <tj@kernel.org>
Date: Sat, 20 Aug 2011 09:31:39 +0000 (+0200)
Subject: freezer: fix current->state restoration race in refrigerator()
X-Git-Tag: next-20110822~18^2^2~15
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=f0043f7496d92de839b0582b0343d2ebc92f9bde;p=karo-tx-linux.git

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

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>
---

diff --git a/kernel/freezer.c b/kernel/freezer.c
index 7b01de98bb6a..575f863a73e9 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -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);