linux-kernel - [PATCH 01/16] freezer: fix current->state restoration race in refrigerator()

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Date:	Fri, 19 Aug 2011 16:16:07 +0200
From:	Tejun Heo <tj@...nel.org>
To:	rjw@...k.pl, menage@...gle.com, linux-kernel@...r.kernel.org
Cc:	arnd@...db.de, oleg@...hat.com, Tejun Heo <tj@...nel.org>
Subject: [PATCH 01/16] 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_sleep();
	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@...nel.org>
---
 kernel/freezer.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/kernel/freezer.c b/kernel/freezer.c
index 7b01de9..575f863 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);
 
-- 
1.7.6

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