Re: [PATCH v5 3.1.0-rc4-tip 18/26] uprobes: slot allocation.

[Srikar Dronamraju]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2011-09-27 14:18:52]:

> On Tue, 2011-09-20 at 17:33 +0530, Srikar Dronamraju wrote:
> > +static struct uprobes_xol_area *xol_alloc_area(void)
> > +{
> > +       struct uprobes_xol_area *area = NULL;
> > +
> > +       area = kzalloc(sizeof(*area), GFP_KERNEL);
> > +       if (unlikely(!area))
> > +               return NULL;
> > +
> > +       area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
> > +                                                               GFP_KERNEL);
> > +
> > +       if (!area->bitmap)
> > +               goto fail;
> > +
> > +       init_waitqueue_head(&area->wq);
> > +       spin_lock_init(&area->slot_lock);
> > +       if (!xol_add_vma(area) && !current->mm->uprobes_xol_area) {
> 
> So what happens if xol_add_vma() succeeds, but we find
> ->uprobes_xol_area set?
> 
> > +               task_lock(current);
> > +               if (!current->mm->uprobes_xol_area) {
> 
> Having to re-test it under this lock seems to suggest it could.
> 
> > +                       current->mm->uprobes_xol_area = area;
> > +                       task_unlock(current);
> > +                       return area;
> 
> This function would be so much easier to read if the success case (this
> here I presume) would not be nested 2 deep.
> 
> > +               }
> > +               task_unlock(current);
> > +       }
> 
> at which point you could end up with two extra vmas? Because there's no
> freeing of the result of xol_add_vma().
> 

Agree, we need to unmap the vma in that case.

> > +fail:
> > +       kfree(area->bitmap);
> > +       kfree(area);
> > +       return current->mm->uprobes_xol_area;
> > +} 

-- 
Thanks and Regards
Srikar

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