Re: [PATCH 1/4] mm/rmap: per anon_vma lock

Yuanhan Liu
2013-11-01T14:09:31Z

On Fri, Nov 01, 2013 at 11:22:24AM +0100, Peter Zijlstra wrote:
> On Fri, Nov 01, 2013 at 05:38:44PM +0800, Yuanhan Liu wrote:
> > On Fri, Nov 01, 2013 at 09:43:29AM +0100, Peter Zijlstra wrote:
> > > On Fri, Nov 01, 2013 at 03:54:24PM +0800, Yuanhan Liu wrote:
> > > > @@ -497,15 +495,20 @@ static void vma_rb_erase(struct vm_area_struct *vma, struct rb_root *root)
> > > > * anon_vma_interval_tree_post_update_vma().
> > > > *
> > > > * The entire update must be protected by exclusive mmap_sem and by
> > > > - * the root anon_vma's mutex.
> > > > + * the anon_vma's mutex.
> > > > */
> > > > static inline void
> > > > anon_vma_interval_tree_pre_update_vma(struct vm_area_struct *vma)
> > > > {
> > > > struct anon_vma_chain *avc;
> > > >
> > > > - list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
> > > > - anon_vma_interval_tree_remove(avc, &avc->anon_vma->rb_root);
> > > > + list_for_each_entry(avc, &vma->anon_vma_chain, same_vma) {
> > > > + struct anon_vma *anon_vma = avc->anon_vma;
> > > > +
> > > > + anon_vma_lock_write(anon_vma);
> > > > + anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
> > > > + anon_vma_unlock_write(anon_vma);
> > > > + }
> > > > }
> > > >
> > > > static inline void
> > > > @@ -513,8 +516,13 @@ anon_vma_interval_tree_post_update_vma(struct vm_area_struct *vma)
> > > > {
> > > > struct anon_vma_chain *avc;
> > > >
> > > > - list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
> > > > - anon_vma_interval_tree_insert(avc, &avc->anon_vma->rb_root);
> > > > + list_for_each_entry(avc, &vma->anon_vma_chain, same_vma) {
> > > > + struct anon_vma *anon_vma = avc->anon_vma;
> > > > +
> > > > + anon_vma_lock_write(anon_vma);
> > > > + anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
> > > > + anon_vma_unlock_write(anon_vma);
> > > > + }
> > > > }
> > > >
> > > > static int find_vma_links(struct mm_struct *mm, unsigned long addr,
> > > > @@ -781,7 +789,6 @@ again: remove_next = 1 + (end > next->vm_end);
> > > > if (anon_vma) {
> > > > VM_BUG_ON(adjust_next && next->anon_vma &&
> > > > anon_vma != next->anon_vma);
> > > > - anon_vma_lock_write(anon_vma);
> > > > anon_vma_interval_tree_pre_update_vma(vma);
> > > > if (adjust_next)
> > > > anon_vma_interval_tree_pre_update_vma(next);
> > > > @@ -845,7 +852,6 @@ again: remove_next = 1 + (end > next->vm_end);
> > > > anon_vma_interval_tree_post_update_vma(vma);
> > > > if (adjust_next)
> > > > anon_vma_interval_tree_post_update_vma(next);
> > > > - anon_vma_unlock_write(anon_vma);
> > > > }
> > > > if (mapping)
> > > > mutex_unlock(&mapping->i_mmap_mutex);
> > >
> > > AFAICT this isn't correct at all. We used to protect the vma interval
> > > tree with the root lock, now we don't.
> >
> > We still use lock to protect anon_vma interval tree, but we lock our own
> > interval tree this time.
>
> Which lock? What protects the chain you're iterating in
> anon_vma_interval_tree_{pre,post}_update_vma() ?

Sorry, I may be wrong again this time. But, isn't vma->anon_vma_chain
list being protect by mmap_sem & page_table_lock?
struct vm_area_struct {
...
struct list_head anon_vma_chain; /* Serialized by mmap_sem &
* page_table_lock */
...
}

So, my understanding was you don't need extra lock to iterate
vma->anon_vma_chain list. However, you need acquire avc->anon_vma's lock
to insert/remove avc from it.

Thanks.

--yliu
>
> > > All we've got left is the
> > > mmap_sem, but anon_vma chains can cross address-spaces and thus we're up
> > > some creek without no paddle.
> >
> > Yep, however, you still need acquire the address-space crossed anon_vma's lock
> > to modify something.
>
> -ENOPARSE.