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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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