Re: [PATCH 3/8] wifi: ath12k: Refactor sta state machine

[Kalle Valo <kvalo@kernel.org>]

Jeff Johnson <quic_jjohnson@quicinc.com> writes:

> On 10/23/2024 6:29 AM, Kalle Valo wrote:
>
>> +static void ath12k_mac_station_post_remove(struct ath12k *ar,
>> +					   struct ath12k_link_vif *arvif,
>> +					   struct ath12k_link_sta *arsta)
>> +{
>> +	struct ieee80211_vif *vif = ath12k_ahvif_to_vif(arvif->ahvif);
>> +	struct ieee80211_sta *sta = ath12k_ahsta_to_sta(arsta->ahsta);
>> +	struct ath12k_sta *ahsta = arsta->ahsta;
>> +	struct ath12k_peer *peer;
>> +
>> +	lockdep_assert_wiphy(ath12k_ar_to_hw(ar)->wiphy);
>> +
>> +	ath12k_mac_dec_num_stations(arvif, arsta);
>> +
>> +	spin_lock_bh(&ar->ab->base_lock);
>> +
>> +	peer = ath12k_peer_find(ar->ab, arvif->vdev_id, sta->addr);
>> +	if (peer && peer->sta == sta) {
>> +		ath12k_warn(ar->ab, "Found peer entry %pM n vdev %i after it was supposedly removed\n",
>> +			    vif->addr, arvif->vdev_id);
>> +		peer->sta = NULL;
>> +		list_del(&peer->list);
>> +		kfree(peer);
>> +		ar->num_peers--;
>> +	}
>> +
>> +	spin_unlock_bh(&ar->ab->base_lock);
>> +
>> +	kfree(arsta->rx_stats);
>> +	arsta->rx_stats = NULL;
>> +
>> +	if (arsta->link_id < IEEE80211_MLD_MAX_NUM_LINKS) {
>> +		rcu_assign_pointer(ahsta->link[arsta->link_id], NULL);
>> +		synchronize_rcu();
>
> I've mentioned this in the past in some internal discussion and seems now is a
> good time to bring this to light.
>
> It concerns me that this happens so late in the process. In theory another
> thread could already have a valid arsta pointer and could be trying to
> dereference that pointer while the code above is destroying underlying data
> (i.e. arsta->rx_stats).
>
> Should we set this to NULL and synchronize RCU at the beginning of the process
> so that we know all access to the struct has finished before we start
> destroying the data?
>
> Or can this not actually happen in practice due to other synchronization
> mechansims? And if so, should we document that somewhere?

I think you are correct, AFAICS the kfree(arsta->rx_stats) should be
after synchronize_rcu(). But this race was already in the code before
this patch so we need to fix in a separate patch. I have added this to
my todo list.

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