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SubjectRe: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way
Date
On Friday, 11 May 2007 21:39, Andrew Morton wrote:
> 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?

For user space processes this condition is always true.

For kernel threads:
(1) the change of tsk->mm from NULL to a nonzero value is only made in
fs/aio.c:use_mm() along with the setting of PF_BORROWED_MM under
the task_lock(),
(2) the change of tsk->mm from a nonzero value to NULL is only made in
fs/aio.c:unuse_mm() along with the resetting of PF_BORROWED_MM
under the task_lock().
Therefore, by taking the task_lock() here we make sure that the condition
is alyways false when we check it for kernel threads.

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