On Friday, 11 May 2007 21:39, Andrew Morton wrote: > On Fri, 11 May 2007 00:36:25 +0200 > "Rafael J. Wysocki" <[EMAIL PROTECTED]> wrote: > > > The reading of PF_BORROWED_MM in is_user_space() without task_lock() is > > racy. > > Fix it. > > > > Signed-off-by: Rafael J. Wysocki <[EMAIL PROTECTED]> > > Acked-by: Pavel Machek <[EMAIL PROTECTED]> > > --- > > 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/