linux-kernel - [PATCH -mm] freezer: Document task_lock in thaw

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Message-Id: <200704210023.19825.rjw@sisk.pl>
Date:	Sat, 21 Apr 2007 00:23:19 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>
Subject: [PATCH -mm] freezer: Document task_lock in thaw_process

From: Rafael J. Wysocki <rjw@...k.pl>

The task_lock() in include/linux/freezer.h:thaw_process() looks as though it
were protecting p->flags, which is not the case.  Add a comment that explains
why it's there.

Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
---
 include/linux/freezer.h |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux-2.6.21-rc6-mm1/include/linux/freezer.h
===================================================================
--- linux-2.6.21-rc6-mm1.orig/include/linux/freezer.h	2007-04-09 15:24:25.000000000 +0200
+++ linux-2.6.21-rc6-mm1/include/linux/freezer.h	2007-04-21 00:17:30.000000000 +0200
@@ -37,6 +37,12 @@ static inline void do_not_freeze(struct 
 
 /*
  * Wake up a frozen process
+ *
+ * task_lock() is taken to prevent the race with refrigerator() which may
+ * occur if the freezing of tasks fails.  Namely, without the lock, if the
+ * freezing of tasks failed, thaw_tasks() might have run before a task in
+ * refrigerator() could call frozen_process(), in which case the task would be
+ * frozen and no one would thaw it.
  */
 static inline int thaw_process(struct task_struct *p)
 {
-
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