Re: [PATCH] atm: Preserve value of skb->truesize when accounting to vcc

quoted

Commit 14afee4b609 ("net: convert sock.sk_wmem_alloc from atomic_t to
refcount_t") did exactly what it was intended to do, and turned this
mostly-theoretical problem into a real one, causing PPPoATM to fail
immediately as sk_wmem_alloc underflows and atm_may_send() *immediately*
starts refusing to allow new packets.

 ...

quoted

Fixes: 14afee4b ("net: convert sock.sk_wmem_alloc from atomic_t to
refcount_t")

This Fixes tag shoots the messenger really.

A little bit, yes. The text hopefully made that clear.

I suggest to instead use :

Fixes: 158f323b9868 ("net: adjust skb->truesize in pskb_expand_head()")

Because even without the conversion to refcount_t, we could have a LOCKDEP
splat in :

filter = rcu_dereference_check(sk->sk_filter,
                               atomic_read(&sk->sk_wmem_alloc) == 0);

Note that some places make a further check even when LOCKDEP is not used.

net/ipv4/af_inet.c:154:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));
net/iucv/af_iucv.c:405:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));
net/key/af_key.c:112:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));
net/netlink/af_netlink.c:410:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));
net/packet/af_packet.c:1286:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));
net/rxrpc/af_rxrpc.c:852:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));
net/unix/af_unix.c:490:	WARN_ON(refcount_read(&sk->sk_wmem_alloc));


We might factorize these checks into __sk_destruct()

How many of those were likely to trigger in practice on an ATM VCC though?
If we are taking the Fixes: tag as a hint about which stable kernels we
might want to backport to, rather than a moral assignment of blame, then
14afee4b is probably not the worst place to point it. But I don't mind
much...

-- 
dwmw2