[PATCH net-next 08/15] batman-adv: tt: annotate last_seen with READ/WRITE_ONCE

[Simon Wunderlich <sw@simonwunderlich.de>]

From: Sven Eckelmann <sven@narfation.org>

The last_seen field for batadv_tt_local_entry tracks the jiffies value of
the most recent activity and is used to detect timeouts. These accesses are
not consistently protected by a lock, so READ_ONCE/WRITE_ONCE must be used
to prevent data races caused by compiler optimizations.

To avoid an underflow due to an non-strict order read of jiffies vs
last_seen, use jiffies_delta_to_msecs instead of jiffies_to_msecs to
evaluate the delta.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
---
 net/batman-adv/translation-table.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c
index 46304069b3b75..76635ae82c84b 100644
--- a/net/batman-adv/translation-table.c
+++ b/net/batman-adv/translation-table.c
@@ -917,7 +917,7 @@ static bool batadv_tt_local_add_existing(struct batadv_priv *bat_priv,
 	struct batadv_tt_common_entry *common = &tt_local->common;
 	bool readded = false;
 
-	tt_local->last_seen = jiffies;
+	WRITE_ONCE(tt_local->last_seen, jiffies);
 
 	scoped_guard(spinlock_bh, &common->flags_lock) {
 		if (common->flags & BATADV_TT_CLIENT_PENDING) {
@@ -971,6 +971,7 @@ batadv_tt_local_create(struct net_device *mesh_iface, const u8 *addr,
 	struct batadv_priv *bat_priv = netdev_priv(mesh_iface);
 	struct batadv_tt_local_entry *tt_local;
 	struct batadv_meshif_vlan *vlan;
+	unsigned long last_seen;
 
 	if (!batadv_tt_local_reserve_entry(bat_priv, addr))
 		return NULL;
@@ -1000,8 +1001,9 @@ batadv_tt_local_create(struct net_device *mesh_iface, const u8 *addr,
 	ether_addr_copy(tt_local->common.addr, addr);
 	tt_local->common.vid = vid;
 	kref_init(&tt_local->common.refcount);
-	tt_local->last_seen = jiffies;
-	tt_local->common.added_at = tt_local->last_seen;
+	last_seen = jiffies;
+	WRITE_ONCE(tt_local->last_seen, last_seen);
+	tt_local->common.added_at = last_seen;
 	tt_local->vlan = vlan;
 	spin_lock_init(&tt_local->common.flags_lock);
 
@@ -1477,12 +1479,14 @@ batadv_tt_local_dump_entry(struct sk_buff *msg, u32 portid,
 	struct batadv_tt_local_entry *local;
 	struct batadv_meshif_vlan *vlan;
 	unsigned int last_seen_msecs;
+	unsigned long last_seen;
 	void *hdr;
 	u16 flags;
 	u32 crc;
 
 	local = container_of(common, struct batadv_tt_local_entry, common);
-	last_seen_msecs = jiffies_to_msecs(jiffies - local->last_seen);
+	last_seen = READ_ONCE(local->last_seen);
+	last_seen_msecs = jiffies_delta_to_msecs(jiffies - last_seen);
 
 	vlan = batadv_meshif_vlan_get(bat_priv, common->vid);
 	if (!vlan)
@@ -1815,7 +1819,8 @@ static void batadv_tt_local_purge_list(struct batadv_priv *bat_priv,
 			if (tt_local_entry->common.flags & BATADV_TT_CLIENT_PENDING)
 				break;
 
-			if (!batadv_has_timed_out(tt_local_entry->last_seen, timeout))
+			if (!batadv_has_timed_out(READ_ONCE(tt_local_entry->last_seen),
+						  timeout))
 				break;
 
 			batadv_tt_local_set_pending(bat_priv, tt_local_entry,
-- 
2.47.3