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SubjectRe: [PATCH - revised] rhashtable: detect when object movement might have invalidated a lookup
On Mon, Jul 16 2018, Herbert Xu wrote:

> On Mon, Jul 16, 2018 at 11:23:43AM +1000, NeilBrown wrote:
>> kmem_cache_free() directly. For this, I need rhashtable to be safe if
>> an object is deleted and immediately re-inserted into the same hash
>> chain.
> This means that
> rcu_read_lock();
> A = rhashtable_lookup();
> use(A);
> rcu_read_unlock();
> A can turn into object B when it is used. That is just too strange
> for words. Can we see some actual code on how this works?

Look in Documenation/RCU/rculist_nulls.txt.
The very first example is a typical lookup for a nulls list.
The above sample code would read:

A = rhashtable_lookup(table, key);
if (IS_ERR(A)) {
goto not_found;
if (!try_get_ref(A))
goto again;
if (A->key != key) {
goto again;

If you particularly need to see real code rather than sample code, I can
have something for you in a day or so.


> For comparison, the existing net code where this happens A doesn't
> actually change and it simply moves from one hashtable to another.
> I'm relucant to add semantics that would restrain on how rhashtable
> works unless we have real and valid use-cases for it.
> Cheers,
> --
> Email: Herbert Xu <>
> Home Page:
> PGP Key:
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