[PATCH 6/7] Freezer: Fix PF_NOFREEZE vs freezeable race

["Rafael J. Wysocki" <rjw@sisk.pl>]

From: Gautham R Shenoy <ego@in.ibm.com>

This patch fixes the race pointed out by Oleg Nesterov.

* Freezer marks a thread as freezeable.
* The thread now marks itself PF_NOFREEZE, but it will be frozen on
  on calling try_to_freeze(). Thus the task is frozen, even though it doesn't
  want to.
* Subsequent thaw_processes() will also fail to thaw the task since it is
  marked PF_NOFREEZE.

Avoid this problem by checking the task's PF_NOFREEZE status in
frozen_processes() before marking the task as frozen.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
 include/linux/freezer.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-2.6/include/linux/freezer.h
===================================================================
--- linux-2.6.orig/include/linux/freezer.h	2007-05-10 21:44:36.000000000 +0200
+++ linux-2.6/include/linux/freezer.h	2007-05-10 21:44:47.000000000 +0200
@@ -63,8 +63,10 @@ static inline int thaw_process(struct ta
  */
 static inline void frozen_process(struct task_struct *p)
 {
-	p->flags |= PF_FROZEN;
-	wmb();
+	if (!unlikely(p->flags & PF_NOFREEZE)) {
+		p->flags |= PF_FROZEN;
+		wmb();
+	}
 	clear_tsk_thread_flag(p, TIF_FREEZE);
 }