On Thursday, 21 November 2024 16:07:24 CET Remi Pommarel wrote:
So the patch would be quite similar, only tt->tt.changes_list_lock will be taken sooner in batadv_tt_tvlv_container_update().
That will fix the ADD between two read situation as you described though.
Do you still prefer this option ?
I can't speak for Antonio but I think I would prefer for the fix the current version. The locking one would end up again with nested spinlocks and maybe more refactoring. And it might be nicer for the stable backports to have less noise in the patch.
Btw. just noticed that the code (not in your change - but overall) for the filling of diff entries could have been something like:
--- a/net/batman-adv/translation-table.c +++ b/net/batman-adv/translation-table.c @@ -980,6 +980,7 @@ static void batadv_tt_tvlv_container_update(struct batadv_priv *bat_priv) int tt_diff_entries_count = 0; bool drop_changes = false; size_t tt_extra_len = 0; + LIST_HEAD(tt_diffs); u16 tvlv_len;
tt_diff_entries_num = READ_ONCE(bat_priv->tt.local_changes); @@ -1011,9 +1012,10 @@ static void batadv_tt_tvlv_container_update(struct batadv_priv *bat_priv)
spin_lock_bh(&bat_priv->tt.changes_list_lock); bat_priv->tt.local_changes = 0; + list_splice_init(&bat_priv->tt.changes_list, &tt_diffs); + spin_unlock_bh(&bat_priv->tt.changes_list_lock);
- list_for_each_entry_safe(entry, safe, &bat_priv->tt.changes_list, - list) { + list_for_each_entry_safe(entry, safe, &tt_diffs, list) { if (tt_diff_entries_count < tt_diff_entries_num) { memcpy(tt_change + tt_diff_entries_count, &entry->change, @@ -1023,7 +1025,6 @@ static void batadv_tt_tvlv_container_update(struct batadv_priv *bat_priv) list_del(&entry->list); kmem_cache_free(batadv_tt_change_cache, entry); } - spin_unlock_bh(&bat_priv->tt.changes_list_lock);
tt_extra_len = batadv_tt_len(tt_diff_entries_num - tt_diff_entries_count);
And then you can also move this before "tt_diff_entries_num = ..." and save the corresponding bat_priv->tt.local_changes for the spliced list to the inside the lock also in a local variable. And then operate on this variable for the other decisions. Of course, you must still clean the local list in case of an error. Which of course would slightly change the behavior in case of an allocation error in batadv_tt_prepare_tvlv_local_data (which would previously kept the list as it was).
But if it would be done like this then we could also remove the READ_ONCE and not introduce the WRITE_ONCE - just because local_changes is only touched inside a locked area (see changes_list_lock).
Please double check these statements - this was just a simple brain dump.
Kind regards, Sven