[B.A.T.M.A.N.] [PATCH v4 03/31] batman-adv: Avoid recursive call_rcu for batadv_bla_claim

Sven Eckelmann
2016-01-05T11:06:18Z

The batadv_hardif_neigh_free_ref function uses call_rcu to delay the free
of the batadv_bla_claim object until no (already started) rcu_read_lock is
enabled anymore. This makes sure that no context is still trying to access
the object which should be removed. But batadv_bla_claim also contains a
reference to backbone_gw which must be removed.

The reference drop of backbone_gw was done in the call_rcu function
batadv_claim_free_rcu but should actually be done in the
batadv_claim_release function to avoid nested call_rcus. This is important
because rcu_barrier (e.g. batadv_softif_free or batadv_exit) will not
detect the inner call_rcu as relevant for its execution. Otherwise this
barrier will most likely be inserted in the queue before the callback of
the first call_rcu was executed. The caller of rcu_barrier will therefore
continue to run before the inner call_rcu callback finished.

Fixes: a9ce0dc43e2c ("batman-adv: add basic bridge loop avoidance code")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
v4:
- fix function names in commit messages
- fix double whitespace in batadv_tt_orig_list_entry_release kerneldoc
- add extra patch for batadv_claim_free_ref kerneldoc fix
- change the phrase "free it" in all *_free_ref/*_put functions to "release it"
v3:
- update copyright year
v2:
- split patchset into fixes and kref migration to make it easier when the
decision is made where each patch will be applied

net/batman-adv/bridge_loop_avoidance.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/net/batman-adv/bridge_loop_avoidance.c b/net/batman-adv/bridge_loop_avoidance.c
index fc1b898..a840d7d 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -154,17 +154,14 @@ batadv_backbone_gw_free_ref(struct batadv_bla_backbone_gw *backbone_gw)
}

/**
- * batadv_claim_free_rcu - finally deinitialize the claim
- * @rcu: rcu pointer within the claim structure
+ * batadv_claim_release - release claim from lists and queue for free after rcu
+ * grace period
+ * @claim: claim to be free'd
*/
-static void batadv_claim_free_rcu(struct rcu_head *rcu)
+static void batadv_claim_release(struct batadv_bla_claim *claim)
{
- struct batadv_bla_claim *claim;
-
- claim = container_of(rcu, struct batadv_bla_claim, rcu);
-
batadv_backbone_gw_free_ref(claim->backbone_gw);
- kfree(claim);
+ kfree_rcu(claim, rcu);
}

/**
@@ -175,7 +172,7 @@ static void batadv_claim_free_rcu(struct rcu_head *rcu)
static void batadv_claim_free_ref(struct batadv_bla_claim *claim)
{
if (atomic_dec_and_test(&claim->refcount))
- call_rcu(&claim->rcu, batadv_claim_free_rcu);
+ batadv_claim_release(claim);
}

/**
--
2.6.4