Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 11 May 2007 00:36:25 +0200
"Rafael J. Wysocki" <rjw@sisk.pl> wrote:

> The reading of PF_BORROWED_MM in is_user_space() without task_lock() is racy. 
> Fix it.
> 
> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
> Acked-by: Pavel Machek <pavel@ucw.cz>
> ---
>  kernel/power/process.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/kernel/power/process.c
> ===================================================================
> --- linux-2.6.orig/kernel/power/process.c	2007-05-10 21:44:23.000000000 +0200
> +++ linux-2.6/kernel/power/process.c	2007-05-10 21:44:28.000000000 +0200
> @@ -8,6 +8,7 @@
>  
>  #undef DEBUG
>  
> +#include <linux/sched.h>
>  #include <linux/interrupt.h>
>  #include <linux/suspend.h>
>  #include <linux/module.h>
> @@ -88,7 +89,12 @@ 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;
>  }

The whole function is racy, isn't it?  I mean, the condition which it is
testing can go from true->false or false->true at any instant after this
function returns its now-wrong value.

iow, callers of this function need to to something to prevent the expression
`p->mm && !(p->flags & PF_BORROWED_MM);' from changing value _anyway_.  In
which case the new locking is not needed?