linux-kernel - Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

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Date:	Sun, 18 Feb 2007 13:12:21 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
	vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>
Subject: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

On Sunday, 18 February 2007 12:32, Oleg Nesterov wrote:
> On 02/18, Rafael J. Wysocki wrote:
> >
> > On Sunday, 18 February 2007 00:42, Oleg Nesterov wrote:
> > > On 02/17, Rafael J. Wysocki wrote:
> > > >
> > > > On Saturday, 17 February 2007 22:34, Oleg Nesterov wrote:
> > > > > 
> > > > > 	static inline int is_user_space(struct task_struct *p)
> > > > > 	{
> > > > > 		return p->mm && !(p->flags & PF_BORROWED_MM);
> > > > > 	}
> > > > > 
> > > > > This doesn't look right. First, an exiting task has ->mm == NULL after
> > > > > do_exit()->exit_mm(). Probably not a problem. However, PF_BORROWED_MM
> > > > > check is racy without task_lock(), so we can have a false positive as
> > > > > well. Is it ok? We can freeze aio_wq prematurely.
> > > > 
> > > > Right now aio_wq is not freezeable (PF_NOFREEZE).
> > > 
> > > Right now yes, but we are going to change this?
> > 
> > Well, is there any more reliable (and not racy) method of differentiating
> > between kernel threads and user space processes?
> 
> Not that I know of. At least, we can take task_lock() to really rule out
> kernel threads at FREEZER_USER_SPACE stage.

Something like this?

---
 kernel/power/process.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

Index: linux-2.6.20-mm2/kernel/power/process.c
===================================================================
--- linux-2.6.20-mm2.orig/kernel/power/process.c
+++ linux-2.6.20-mm2/kernel/power/process.c
@@ -8,6 +8,7 @@
 
 #undef DEBUG
 
+#include <linux/sched.h>
 #include <linux/smp_lock.h>
 #include <linux/interrupt.h>
 #include <linux/suspend.h>
@@ -92,7 +93,12 @@ static void cancel_freezing(struct task_
 
 static inline int is_user_space(struct task_struct *p)
 {
-	return p->mm && !(p->flags & PF_BORROWED_MM);
+	int ret;
+
+	task_lock(p);
+	ret = p->mm && !(p->flags & PF_BORROWED_MM);
+	task_unlock(p);
+	return ret;
 }
 
 static unsigned int try_to_freeze_tasks(int freeze_user_space)
-
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