[RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race

[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 causing it to
  freeze 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 current task's PF_NOFREEZE status in the 
refrigerator before marking current as frozen.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
---
 kernel/power/process.c |    9 +++++++++
 1 file changed, 9 insertions(+)

Index: linux-2.6.21-rc6/kernel/power/process.c
===================================================================
--- linux-2.6.21-rc6.orig/kernel/power/process.c
+++ linux-2.6.21-rc6/kernel/power/process.c
@@ -41,6 +41,15 @@ void refrigerator(void)
 
 	task_lock(current);
 	if (freezing(current)) {
+		/* check if we had marked ourself PF_NOFREEZE
+		 * *after* the freezer did the freezeable() check
+		 * on us.
+		 */
+		if (current->flags & PF_NOFREEZE) {
+			clear_tsk_thread_flag(current, TIF_FREEZE);
+			task_unlock(current);
+			return;
+		}
 		frozen_process(current);
 		task_unlock(current);
 	} else {
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"