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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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