Re: [PATCH 1/4] rhashtable: use cmpxchg() in nested_table_alloc()

Herbert Xu
2019-04-08T02:34:55Z

On Tue, Apr 02, 2019 at 10:07:45AM +1100, NeilBrown wrote:
> nested_table_alloc() relies on the fact that there is
> at most one spinlock allocated for every slot in the top
> level nested table, so it is not possible for two threads
> to try to allocate the same table at the same time.
>
> This assumption is a little fragile (it is not explicit) and is
> unnecessary as cmpxchg() can be used instead.
>
> A future patch will replace the spinlocks by per-bucket bitlocks,
> and then we won't be able to protect the slot pointer with a spinlock.
>
> So replace rcu_assign_pointer() with cmpxchg() - which has equivalent
> barrier properties.
> If it the cmp fails, free the table that was just allocated.
>
> Signed-off-by: NeilBrown <neilb@suse.com>

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt