Re: PREEMPT_RCU breaks anon_vma locking ?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Feb 24, 2007 at 12:23:03AM +0300, Oleg Nesterov wrote:
> If my understanding correct, vmscan can find a page which lives in a already
> anon_vma_unlink'ed vma. This is ok, the page is pinned, and page->mapping is
> not cleared until free_hot_cold_page().
> 
> So page_lock_anon_vma() works correctly due to SLAB_DESTROY_BY_RCU even if
> anon_vma_unlink() has already freed anon_vma. In that case we should see
> list_empty(&anon_vma->head), we are safe.
> 
> However, we are doing spin_unlock(anon_vma->lock) after page_lock_anon_vma(),
> and this looks unsafe to me because page_lock_anon_vma() does rcu_read_unlock()
> on return.

This would indeed be bad when using CONFIG_PREEMPT_RCU!  Good catch!!!

> This worked before because spin_lock() implied rcu_read_lock(), so rcu was
> blocked if page_lock_anon_vma() returns !NULL. With CONFIG_PREEMPT_RCU this
> is not true (yes?), so it is possible that the slab returns the memory to
> the system and it is re-used when we write to anon_vma->lock.
> 
> IOW, don't we need something like this
> 
> 	static struct anon_vma *page_lock_anon_vma(struct page *page)
> 	{
> 		struct anon_vma *anon_vma;
> 		unsigned long anon_mapping;
> 
> 		rcu_read_lock();
> 		anon_mapping = (unsigned long) page->mapping;
> 		if (!(anon_mapping & PAGE_MAPPING_ANON))
> 			goto out;
> 		if (!page_mapped(page))
> 			goto out;
> 
> 		anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> 		spin_lock(&anon_vma->lock);
> 		return anon_vma;
> 
> 	out:
> 		rcu_read_unlock();
> 		return NULL;
> 	}
> 
> 	static inline void page_lock_anon_vma(struct anon_vma *anon_vma)
> 	{
> 		spin_unlock(&anon_vma->lock);
> 		rcu_read_unlock();
> 	}
> ?

This look like a valid fix to me, at least as long as the lock is never
dropped in the meantime (e.g., to do I/O).  If the lock -is- dropped in
the meantime, then presumably whatever is done to keep the page from
vanishing should allow an rcu_read_unlock() to be placed after each
spin_unlock(&...->lock) and an rcu_read_lock() to be placed before each
spin_lock(&...->lock).

						Thanx, Paul
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