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





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