Re: [PATCH net v2 12/14] batman-adv: tt: only update flags for TT events

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- [Medium] The patch now treats each new TT event as the client's latest
  state and writes its flags over any queued change for the same…
- [Low] Claimed mismatch between the kernel-doc and the new behaviour for
  NEW clients.

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batman-adv: tt: only update flags for TT events

This commit changes __batadv_tt_local_event(). DEL+ADD and ADD+DEL pairs
for the same client no longer cancel each other. Instead, a new event
overwrites the flags of the change that is already queued. This keeps a
DEL that follows a flag-update ADD, and it keeps an ADD with new flags
that follows a DEL.

[ ... ]

[Severity: Low]

This isn't a bug, but for a client still in the NEW state, the ADD+DEL
pair used to cancel. It is now kept as an announced DEL:

batadv_tt_local_remove()
  batadv_tt_local_mark_removed()   <- breaks out for BATADV_TT_CLIENT_NEW
  batadv_tt_local_remove_now()
    batadv_tt_local_event(bat_priv, tt_local_entry, BATADV_TT_CLIENT_DEL);

When a NEW client is added and removed within one OGM interval, this
causes an extra TTVN increment. It also puts a DEL entry in the TT diff
for a client that was never announced.

Receivers seem to treat that DEL as a no-op, because batadv_tt_global_del()
bails out when batadv_tt_global_hash_find() finds nothing. The commit
message also says the extra event is accepted for now ("The first and the
last type of event could in theory still be removed together ...").

Is the extra announcement intended for the NEW-client case, or would it be
worth keeping the cancellation there?

[Severity: Medium]

Does this overwrite rely on events reaching changes_list_lock in the same
order as the state changes they describe?

batadv_tt_local_event() takes its flags snapshot under flags_lock. It
releases that lock before __batadv_tt_local_event() takes
changes_list_lock:

net/batman-adv/translation-table.c:batadv_tt_local_event() {
	...
	flags = batadv_tt_flags_get(common) | event_flags;
	__batadv_tt_local_event(bat_priv, common, flags);
}

The ADD callers in batadv_tt_local_add() hold neither the bucket list_lock
nor flags_lock here. The DEL paths queue their event either under those
locks (batadv_tt_local_set_pending()) or after removing the entry from the
hash (batadv_tt_local_remove_now()). Nothing orders either DEL path against
a concurrent ADD.

Here is the sequence for a new client:

CPU A (TX path)
batadv_interface_tx()
  batadv_tt_local_add()
    batadv_tt_local_create()
    batadv_hash_add()             <- entry is now visible
    batadv_tt_local_event(ADD)
      batadv_tt_flags_get()
                                  CPU B (RX path)
                                  batadv_tt_global_add()
                                    batadv_tt_global_purge_local()
                                      batadv_tt_local_remove()
                                        batadv_tt_local_mark_removed()
                                          <- NEW, not set pending
                                        batadv_tt_local_remove_now()
                                          batadv_hash_remove()
                                          batadv_tt_local_event(DEL)
      __batadv_tt_local_event()
        entry->change.flags = flags;  <- queued DEL becomes ADD

Before this patch the DEL+ADD pair cancelled. That gave the right result
in this case, because the client was never announced and is no longer in
the local hash.

With this change, can an ADD be announced for a client that is no longer
in the local table?

The same window exists for an existing client. batadv_tt_local_add() can
take the ADD snapshot after batadv_tt_local_update_flags(), and
batadv_tt_local_set_pending() can then queue DEL|... before the ADD gets
changes_list_lock. The DEL was also lost in this ordering before the
patch. Now the stale ADD replaces it as the latest state.

In both cases, remote global tables would keep a client that the local
node removes, through batadv_tt_local_purge_pending_clients() at the next
commit in the second case. This lasts until a CRC mismatch triggers a full
table request.

As far as I can tell, the later patches in this series don't change
batadv_tt_local_event() or __batadv_tt_local_event(), so the window is
still there at the end of the series.

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261030135105.2555887-2-sw%40simonwunderlich.de