[B.A.T.M.A.N.] [PATCH] Re: batman-adv: Correct rcu refcounting for gw_node

Linus Lüssing
2011-02-02T17:37:18Z

From: Sven Eckelmann <sven@narfation.org>

Was:
---
<TODO: write a long monologue about every problem we have or could have or
maybe never had and would have when we not have it>

Signed-off-by: Sven Eckelmann <sven@narfation.org>
---

So after some more discussions with Marek and Sven, it looks like we
have to use the rcu protected macros rcu_dereference() and
rcu_assign_pointer() for the bat_priv->curr_gw and curr_gw->orig_node.

Changes here also include moving the kref_get() from unicast_send_skb()
into gw_get_selected(). The orig_node could have been freed already at
the time the kref_get() was called in unicast_send_skb().

Some things that are still not that clear to me:

gw_election():
* can the if-block before gw_deselect() be ommited, we had a nullpointer
check for curr_gw just a couple of lines before during the rcu-lock.

gw_deselet():
* is the refcount at this time always 1 for gw_node, can the null
pointer check + a rcu_dereference be ommited? (at least that's what
it looks like when comparing to the rcuref.txt example)

gw_get_selected():
* Probably the orig_node's refcounting has to be made atomic, too?

Cheers, Linus

Not-Signed-off-by: Linus Lüssing <linus.luessing@ascom.ch>
---
gateway_client.c | 169 +++++++++++++++++++++++++++++++++---------------------
main.c | 1 +
types.h | 7 +-
unicast.c | 1 -
4 files changed, 109 insertions(+), 69 deletions(-)

diff --git a/gateway_client.c b/gateway_client.c
index 429a013..96a67bc 100644
--- a/gateway_client.c
+++ b/gateway_client.c
@@ -28,40 +28,54 @@
#include <linux/udp.h>
#include <linux/if_vlan.h>

-static void gw_node_free_ref(struct kref *refcount)
+static void gw_node_free_rcu(struct rcu_head *rcu)
{
struct gw_node *gw_node;

- gw_node = container_of(refcount, struct gw_node, refcount);
+ gw_node = container_of(rcu, struct gw_node, rcu);
kfree(gw_node);
}

-static void gw_node_free_rcu(struct rcu_head *rcu)
+static void gw_node_free_ref(struct gw_node *gw_node)
{
- struct gw_node *gw_node;
-
- gw_node = container_of(rcu, struct gw_node, rcu);
- kref_put(&gw_node->refcount, gw_node_free_ref);
+ if (atomic_dec_and_test(&gw_node->refcount))
+ call_rcu(&gw_node->rcu, gw_node_free_rcu);
}

+/* increases the returned orig_node's refcount */
void *gw_get_selected(struct bat_priv *bat_priv)
{
- struct gw_node *curr_gateway_tmp = bat_priv->curr_gw;
+ struct gw_node *curr_gateway_tmp;
+ struct orig_node *orig_node;

- if (!curr_gateway_tmp)
+ rcu_read_lock();
+ curr_gateway_tmp = rcu_dereference(bat_priv->curr_gw);
+ if (!curr_gateway_tmp) {
+ rcu_read_unlock();
return NULL;
+ }

- return curr_gateway_tmp->orig_node;
+ orig_node = rcu_dereference(curr_gateway_tmp->orig_node);
+ if (orig_node) {
+ kref_get(&orig_node->refcount);
+ rcu_read_unlock();
+ return NULL;
+ }
+
+ rcu_read_unlock();
+ return orig_node;
}

void gw_deselect(struct bat_priv *bat_priv)
{
- struct gw_node *gw_node = bat_priv->curr_gw;
+ struct gw_node *gw_node;

- bat_priv->curr_gw = NULL;
+ spin_lock_bh(&bat_priv->curr_gw_lock);
+ gw_node = bat_priv->curr_gw;
+ rcu_assign_pointer(bat_priv->curr_gw, NULL);
+ spin_unlock_bh(&bat_priv->curr_gw_lock);

- if (gw_node)
- kref_put(&gw_node->refcount, gw_node_free_ref);
+ gw_node_free_ref(gw_node);
}

static struct gw_node *gw_select(struct bat_priv *bat_priv,
@@ -69,17 +83,21 @@ static struct gw_node *gw_select(struct bat_priv *bat_priv,
{
struct gw_node *curr_gw_node = bat_priv->curr_gw;

- if (new_gw_node)
- kref_get(&new_gw_node->refcount);
+ if (new_gw_node && !atomic_inc_not_zero(&new_gw_node->refcount))
+ return NULL;
+
+ spin_lock_bh(&bat_priv->curr_gw_lock);
+ rcu_assign_pointer(bat_priv->curr_gw, new_gw_node);
+ spin_unlock_bh(&bat_priv->curr_gw_lock);

- bat_priv->curr_gw = new_gw_node;
return curr_gw_node;
}

void gw_election(struct bat_priv *bat_priv)
{
struct hlist_node *node;
- struct gw_node *gw_node, *curr_gw_tmp = NULL, *old_gw_node = NULL;
+ struct gw_node *gw_node, *curr_gw, *curr_gw_tmp = NULL, *old_gw_node = NULL;
+ struct orig_node *orig_node;
uint8_t max_tq = 0;
uint32_t max_gw_factor = 0, tmp_gw_factor = 0;
int down, up;
@@ -93,25 +111,28 @@ void gw_election(struct bat_priv *bat_priv)
if (atomic_read(&bat_priv->gw_mode) != GW_MODE_CLIENT)
return;

- if (bat_priv->curr_gw)
- return;
-
rcu_read_lock();
- if (hlist_empty(&bat_priv->gw_list)) {
+ curr_gw = rcu_dereference(bat_priv->curr_gw);
+ if (curr_gw) {
rcu_read_unlock();
+ return;
+ }

- if (bat_priv->curr_gw) {
+ if (hlist_empty(&bat_priv->gw_list)) {
+ if (curr_gw) {
bat_dbg(DBG_BATMAN, bat_priv,
"Removing selected gateway - "
"no gateway in range\n");
gw_deselect(bat_priv);
}
+ rcu_read_unlock();

return;
}

hlist_for_each_entry_rcu(gw_node, node, &bat_priv->gw_list, list) {
- if (!gw_node->orig_node->router)
+ orig_node = rcu_dereference(gw_node->orig_node);
+ if (!orig_node->router)
continue;

if (gw_node->deleted)
@@ -119,18 +140,17 @@ void gw_election(struct bat_priv *bat_priv)

switch (atomic_read(&bat_priv->gw_sel_class)) {
case 1: /* fast connection */
- gw_bandwidth_to_kbit(gw_node->orig_node->gw_flags,
- &down, &up);
+ gw_bandwidth_to_kbit(orig_node->gw_flags, &down, &up);

- tmp_gw_factor = (gw_node->orig_node->router->tq_avg *
- gw_node->orig_node->router->tq_avg *
+ tmp_gw_factor = (orig_node->router->tq_avg *
+ orig_node->router->tq_avg *
down * 100 * 100) /
(TQ_LOCAL_WINDOW_SIZE *
TQ_LOCAL_WINDOW_SIZE * 64);

if ((tmp_gw_factor > max_gw_factor) ||
((tmp_gw_factor == max_gw_factor) &&
- (gw_node->orig_node->router->tq_avg > max_tq)))
+ (orig_node->router->tq_avg > max_tq)))
curr_gw_tmp = gw_node;
break;

@@ -142,37 +162,38 @@ void gw_election(struct bat_priv *bat_priv)
* soon as a better gateway appears which has
* $routing_class more tq points)
**/
- if (gw_node->orig_node->router->tq_avg > max_tq)
+ if (orig_node->router->tq_avg > max_tq)
curr_gw_tmp = gw_node;
break;
}

- if (gw_node->orig_node->router->tq_avg > max_tq)
- max_tq = gw_node->orig_node->router->tq_avg;
+ if (orig_node->router->tq_avg > max_tq)
+ max_tq = orig_node->router->tq_avg;

if (tmp_gw_factor > max_gw_factor)
max_gw_factor = tmp_gw_factor;
}

- if (bat_priv->curr_gw != curr_gw_tmp) {
- if ((bat_priv->curr_gw) && (!curr_gw_tmp))
+ if (curr_gw != curr_gw_tmp) {
+ orig_node = rcu_dereference(curr_gw_tmp->orig_node);
+ if ((curr_gw) && (!curr_gw_tmp))
bat_dbg(DBG_BATMAN, bat_priv,
"Removing selected gateway - "
"no gateway in range\n");
- else if ((!bat_priv->curr_gw) && (curr_gw_tmp))
+ else if ((!curr_gw) && (curr_gw_tmp))
bat_dbg(DBG_BATMAN, bat_priv,
"Adding route to gateway %pM "
"(gw_flags: %i, tq: %i)\n",
- curr_gw_tmp->orig_node->orig,
- curr_gw_tmp->orig_node->gw_flags,
- curr_gw_tmp->orig_node->router->tq_avg);
+ orig_node->orig,
+ orig_node->gw_flags,
+ orig_node->router->tq_avg);
else
bat_dbg(DBG_BATMAN, bat_priv,
"Changing route to gateway %pM "
"(gw_flags: %i, tq: %i)\n",
- curr_gw_tmp->orig_node->orig,
- curr_gw_tmp->orig_node->gw_flags,
- curr_gw_tmp->orig_node->router->tq_avg);
+ orig_node->orig,
+ orig_node->gw_flags,
+ orig_node->router->tq_avg);

old_gw_node = gw_select(bat_priv, curr_gw_tmp);
}
@@ -181,36 +202,40 @@ void gw_election(struct bat_priv *bat_priv)

/* the kfree() has to be outside of the rcu lock */
if (old_gw_node)
- kref_put(&old_gw_node->refcount, gw_node_free_ref);
+ gw_node_free_ref(old_gw_node);
}

void gw_check_election(struct bat_priv *bat_priv, struct orig_node *orig_node)
{
- struct gw_node *curr_gateway_tmp = bat_priv->curr_gw;
+ struct gw_node *curr_gateway_tmp;
+ struct orig_node *curr_gw_orig;
uint8_t gw_tq_avg, orig_tq_avg;

+ rcu_read_lock();
+ curr_gateway_tmp = rcu_dereference(bat_priv->curr_gw);
if (!curr_gateway_tmp)
- return;
+ goto rcu_unlock;

- if (!curr_gateway_tmp->orig_node)
+ curr_gw_orig = rcu_dereference(curr_gateway_tmp->orig_node);
+ if (!curr_gw_orig)
goto deselect;

- if (!curr_gateway_tmp->orig_node->router)
+ if (!curr_gw_orig->router)
goto deselect;

/* this node already is the gateway */
- if (curr_gateway_tmp->orig_node == orig_node)
- return;
+ if (curr_gw_orig == orig_node)
+ goto deselect;

if (!orig_node->router)
- return;
+ goto rcu_unlock;

- gw_tq_avg = curr_gateway_tmp->orig_node->router->tq_avg;
+ gw_tq_avg = curr_gw_orig ->router->tq_avg;
orig_tq_avg = orig_node->router->tq_avg;

/* the TQ value has to be better */
if (orig_tq_avg < gw_tq_avg)
- return;
+ goto rcu_unlock;

/**
* if the routing class is greater than 3 the value tells us how much
@@ -218,7 +243,7 @@ void gw_check_election(struct bat_priv *bat_priv, struct orig_node *orig_node)
**/
if ((atomic_read(&bat_priv->gw_sel_class) > 3) &&
(orig_tq_avg - gw_tq_avg < atomic_read(&bat_priv->gw_sel_class)))
- return;
+ goto rcu_unlock;

bat_dbg(DBG_BATMAN, bat_priv,
"Restarting gateway selection: better gateway found (tq curr: "
@@ -227,6 +252,8 @@ void gw_check_election(struct bat_priv *bat_priv, struct orig_node *orig_node)

deselect:
gw_deselect(bat_priv);
+rcu_unlock:
+ rcu_read_unlock();
}

static void gw_node_add(struct bat_priv *bat_priv,
@@ -242,7 +269,7 @@ static void gw_node_add(struct bat_priv *bat_priv,
memset(gw_node, 0, sizeof(struct gw_node));
INIT_HLIST_NODE(&gw_node->list);
gw_node->orig_node = orig_node;
- kref_init(&gw_node->refcount);
+ atomic_set(&gw_node->refcount, 1);

spin_lock_bh(&bat_priv->gw_list_lock);
hlist_add_head_rcu(&gw_node->list, &bat_priv->gw_list);
@@ -325,7 +352,7 @@ void gw_node_purge(struct bat_priv *bat_priv)
gw_deselect(bat_priv);

hlist_del_rcu(&gw_node->list);
- call_rcu(&gw_node->rcu, gw_node_free_rcu);
+ gw_node_free_ref(gw_node);
}

@@ -335,21 +362,29 @@ void gw_node_purge(struct bat_priv *bat_priv)
static int _write_buffer_text(struct bat_priv *bat_priv,
struct seq_file *seq, struct gw_node *gw_node)
{
- int down, up;
+ struct gw_node *curr_gw;
+ struct orig_node *orig_node;
+ int down, up, ret;

- gw_bandwidth_to_kbit(gw_node->orig_node->gw_flags, &down, &up);
-
- return seq_printf(seq, "%s %pM (%3i) %pM [%10s]: %3i - %i%s/%i%s\n",
- (bat_priv->curr_gw == gw_node ? "=>" : " "),
- gw_node->orig_node->orig,
- gw_node->orig_node->router->tq_avg,
- gw_node->orig_node->router->addr,
- gw_node->orig_node->router->if_incoming->net_dev->name,
- gw_node->orig_node->gw_flags,
+ rcu_read_lock();
+ curr_gw = rcu_dereference(bat_priv->curr_gw);
+ orig_node = rcu_dereference(gw_node->orig_node);
+ gw_bandwidth_to_kbit(orig_node->gw_flags, &down, &up);
+
+ ret = seq_printf(seq, "%s %pM (%3i) %pM [%10s]: %3i - %i%s/%i%s\n",
+ (curr_gw == gw_node ? "=>" : " "),
+ orig_node->orig,
+ orig_node->router->tq_avg,
+ orig_node->router->addr,
+ orig_node->router->if_incoming->net_dev->name,
+ orig_node->gw_flags,
(down > 2048 ? down / 1024 : down),
(down > 2048 ? "MBit" : "KBit"),
(up > 2048 ? up / 1024 : up),
(up > 2048 ? "MBit" : "KBit"));
+ rcu_read_unlock();
+
+ return ret;
}

int gw_client_seq_print_text(struct seq_file *seq, void *offset)
@@ -470,8 +505,12 @@ int gw_is_target(struct bat_priv *bat_priv, struct sk_buff *skb)
if (atomic_read(&bat_priv->gw_mode) == GW_MODE_SERVER)
return -1;

- if (!bat_priv->curr_gw)
+ rcu_read_lock();
+ if (!rcu_dereference(bat_priv->curr_gw)) {
+ rcu_read_unlock();
return 0;
+ }
+ rcu_read_unlock();

return 1;
}
diff --git a/main.c b/main.c
index e687e7f..8679260 100644
--- a/main.c
+++ b/main.c
@@ -85,6 +85,7 @@ int mesh_init(struct net_device *soft_iface)
spin_lock_init(&bat_priv->hna_lhash_lock);
spin_lock_init(&bat_priv->hna_ghash_lock);
spin_lock_init(&bat_priv->gw_list_lock);
+ spin_lock_init(&bat_priv->curr_gw_lock);
spin_lock_init(&bat_priv->vis_hash_lock);
spin_lock_init(&bat_priv->vis_list_lock);
spin_lock_init(&bat_priv->softif_neigh_lock);
diff --git a/types.h b/types.h
index e4a0462..b9b20b6 100644
--- a/types.h
+++ b/types.h
@@ -98,9 +98,9 @@ struct orig_node {

struct gw_node {
struct hlist_node list;
- struct orig_node *orig_node;
+ struct orig_node *orig_node; /* rcu protected pointer */
unsigned long deleted;
- struct kref refcount;
+ atomic_t refcount;
struct rcu_head rcu;
};

@@ -163,6 +163,7 @@ struct bat_priv {
spinlock_t hna_lhash_lock; /* protects hna_local_hash */
spinlock_t hna_ghash_lock; /* protects hna_global_hash */
spinlock_t gw_list_lock; /* protects gw_list */
+ spinlock_t curr_gw_lock; /* protects curr_gw updates */
spinlock_t vis_hash_lock; /* protects vis_hash */
spinlock_t vis_list_lock; /* protects vis_info::recv_list */
spinlock_t softif_neigh_lock; /* protects soft-interface neigh list */
@@ -171,7 +172,7 @@ struct bat_priv {
struct delayed_work hna_work;
struct delayed_work orig_work;
struct delayed_work vis_work;
- struct gw_node *curr_gw;
+ struct gw_node *curr_gw; /* rcu protected pointer */
struct vis_info *my_vis_info;
};

diff --git a/unicast.c b/unicast.c
index 6a9ab61..8816102 100644
--- a/unicast.c
+++ b/unicast.c
@@ -298,7 +298,6 @@ int unicast_send_skb(struct sk_buff *skb, struct bat_priv *bat_priv)
if (!orig_node)
goto trans_search;

- kref_get(&orig_node->refcount);
goto find_router;
} else {
rcu_read_lock();
--
1.7.2.3