On Fri, 2015-12-18 at 14:24 +0800, Herbert Xu wrote:
> On Fri, Dec 18, 2015 at 01:34:16PM +0800, Herbert Xu wrote:
> > On Fri, Dec 18, 2015 at 09:39:22AM +0800, kernel test robot wrote:
> > > FYI, we noticed the below changes on
> > >
> > > https://github.com/0day-ci/linux
> > > Herbert-Xu/rhashtable-Fix-walker-list-corruption/20151216-164833
> > > commit f9f51b8070be3e829100614a7372b219723b864f ("rhashtable: Fix walker
> > > list corruption")
> > >
> > > [ 8.933376] ===============================
> > > [ 8.933376] ===============================
> > > [ 8.934629] [ INFO: suspicious RCU usage. ]
> > > [ 8.934629] [ INFO: suspicious RCU usage. ]
> > > [ 8.935941] 4.4.0-rc3-00995-gf9f51b8 #2 Not tainted
> > > [ 8.935941] 4.4.0-rc3-00995-gf9f51b8 #2 Not tainted
> > > [ 8.937494] -------------------------------
> > > [ 8.937494] -------------------------------
> > > [ 8.938818] lib/rhashtable.c:504 suspicious
> > > rcu_dereference_protected() usage!
> > > [ 8.938818] lib/rhashtable.c:504 suspicious
> > > rcu_dereference_protected() usage!
> >
> > This is actually a false positive because the new spin lock that
> > we hold prevents ht->tbl from disappearing under us. So here is
> > a patch to kill the warning with a comment.
>
> Resent with a proper patch subject and reported-by.
>
> ---8<---
> The commit f9f51b8070be3e829100614a7372b219723b864f ("rhashtable:
> Fix walker list corruption") causes a suspicious RCU usage warning
> because we no longer hold ht->mutex when we dereference ht->tbl.
>
> However, this is a false positive because we now hold ht->lock
> which also guarantees that ht->tbl won't disppear from under us.
>
> This patch kills the warning by using rcu_dereference_raw and
> adding a comment.
>
> Reported-by: kernel test robot <ying.hu...@linux.intel.com>
> Signed-off-by: Herbert Xu <herb...@gondor.apana.org.au>
>
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index eb9240c..3404b06 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -519,7 +519,11 @@ int rhashtable_walk_init(struct rhashtable *ht, struct
> rhashtable_iter *iter)
> return -ENOMEM;
>
> spin_lock(&ht->lock);
> - iter->walker->tbl = rht_dereference(ht->tbl, ht);
> + /* We do not need RCU protection because we hold ht->lock
> + * which guarantees that if we see ht->tbl then it won't
> + * die on us.
> + */
> + iter->walker->tbl = rcu_dereference_raw(ht->tbl);
You can avoid the comment by using the self documented and lockdep
enabled primitive
iter->walker->tbl = rcu_dereference_protected(ht->tbl,
lockdep_is_held(&ht->lock));
But, storing the ht->tbl and then releasing the lock immediately after
escapes RCU protection.
So why do we store ht->tbl in the first place ?
What exactly prevents it from disappearing after lock is released ?
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
