linux-kernel - [PATCH pm-freezer 2/4] freezer: set PF_NOFREEZE on a dying task right before TASK_DEAD setting bug in freezer_change

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Message-ID: <20110831102143.GB2828@mtj.dyndns.org>
Date:	Wed, 31 Aug 2011 12:21:43 +0200
From:	Tejun Heo <tj@...nel.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>, Oleg Nesterov <oleg@...hat.com>,
	Paul Menage <paul@...lmenage.org>
Cc:	containers@...ts.linux-foundation.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: [PATCH pm-freezer 2/4] freezer: set PF_NOFREEZE on a dying task
 right before TASK_DEAD setting bug in freezer_change_state()

3fb45733df "freezer: make exiting tasks properly unfreezable" removed
clear_freeze_flag() from exit path and set PF_NOFREEZE right after
PTRACE_EVENT_EXIT; however, Oleg pointed out that following exit paths
may cause interaction with device drivers after PM freezer consider
the system frozen.

There's no try_to_freeze() call in the exit path and the only
necessary guarantee is that freezer doesn't hang waiting for zombies.
Set PF_NOFREEZE right before setting tsk->state to TASK_DEAD instead.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Oleg Nesterov <oleg@...hat.com>
Cc: "Rafael J. Wysocki" <rjw@...k.pl>
---
 kernel/exit.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

Index: work/kernel/exit.c
===================================================================
--- work.orig/kernel/exit.c
+++ work/kernel/exit.c
@@ -913,12 +913,6 @@ NORET_TYPE void do_exit(long code)
 
 	ptrace_event(PTRACE_EVENT_EXIT, code);
 
-	/*
-	 * With ptrace notification done, there's no point in freezing from
-	 * here on.  Disallow freezing.
-	 */
-	current->flags |= PF_NOFREEZE;
-
 	validate_creds_for_do_exit(tsk);
 
 	/*
@@ -1044,6 +1038,10 @@ NORET_TYPE void do_exit(long code)
 
 	preempt_disable();
 	exit_rcu();
+
+	/* this task is now dead and freezer should ignore it */
+	current->flags |= PF_NOFREEZE;
+
 	/* causes final put_task_struct in finish_task_switch(). */
 	tsk->state = TASK_DEAD;
 	schedule();
--
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