Re: [PATCH v2 2.6.38-rc8-tip 7/20] 7: uprobes: store/restore original instruction.

Peter Zijlstra
2011-03-15T17:58:31Z

On Tue, 2011-03-15 at 14:52 +0530, Srikar Dronamraju wrote:
> * Stephen Wilson <wilsons@start.ca> [2011-03-14 14:09:14]:
>
> > On Mon, Mar 14, 2011 at 07:05:22PM +0530, Srikar Dronamraju wrote:
> > > static int install_uprobe(struct mm_struct *mm, struct uprobe *uprobe)
> > > {
> > > - int ret = 0;
> > > + struct task_struct *tsk;
> > > + int ret = -EINVAL;
> > >
> > > - /*TODO: install breakpoint */
> > > - if (!ret)
> > > + get_task_struct(mm->owner);
> > > + tsk = mm->owner;
> > > + if (!tsk)
> > > + return ret;
> >
> > I think you need to check that tsk != NULL before calling
> > get_task_struct()...
> >
>
> Guess checking for tsk != NULL would only help if and only if we are doing
> within rcu. i.e we have to change to something like this
>
> rcu_read_lock()
> if (mm->owner) {
> get_task_struct(mm->owner)
> tsk = mm->owner;
> }
> rcu_read_unlock()
> if (!tsk)
> return ret;

so the whole mm->owner semantics seem vague, memcontrol.c doesn't seem
consistent in itself, one site uses rcu_dereference() the other site
doesn't.

Also, the assignments in kernel/fork.c and kernel/exit.c don't use
rcu_assign_pointer() and therefore lack the needed write barrier.

Git blames Balbir for this.