Re: [PATCH 5/6] IMA: use rbtree instead of radix tree for inode information cache

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2010-10-20 at 10:17 +1100, Dave Chinner wrote:
> On Tue, Oct 19, 2010 at 06:58:39PM -0400, Eric Paris wrote:
> > @@ -36,12 +63,11 @@ struct ima_iint_cache *ima_iint_find_get(struct inode *inode)
> >  	struct ima_iint_cache *iint;
> >  
> >  	rcu_read_lock();
> > -	iint = radix_tree_lookup(&ima_iint_store, (unsigned long)inode);
> > -	if (!iint)
> > -		goto out;
> > -	kref_get(&iint->refcount);
> > -out:
> > +	iint = __ima_iint_find(inode);
> > +	if (iint)
> > +		kref_get(&iint->refcount);
> >  	rcu_read_unlock();
> > +
> 
> This is wrong - the rbtree is protected only by the ima_iint_lock(),
> not RCU. Hence you can't do lockless lookups on an rbtree in this
> manner as they will race with inserts and deletes.

Correct, what can be made to work is combine RCU with a seqlock. Retry
the lookup using read_seqretry(), RCU here helps to ensure you're not
stepping on already freed memory.


So, tree modification does:

  write_seqlock();
  /* frob RB-tree, using call_rcu() for frees where needed */
  write_sequnlock();

Lookup does:

  unsigned seq;

  rcu_read_lock()
again;
  seq = read_seqbegin();

  /* RB-tree lookup */

  if (read_seqretry(seq))
    goto again;

  rcu_read_unlock();

  return obj;