Re: [PATCH net-next] packet: fix warnings in rollover lock contention

[Willem de Bruijn <willemb@google.com>]

>> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
>> index 31d5856..ac1a589 100644
>> --- a/net/packet/af_packet.c
>> +++ b/net/packet/af_packet.c
>> @@ -1310,8 +1310,7 @@ static int packet_rcv_has_room(struct packet_sock *po, struct sk_buff *skb)
>>       }
>>
>>       has_room = ret == ROOM_NORMAL;
>> -     if (po->pressure == has_room)
>> -             xchg(&po->pressure, !has_room);
>> +     if (atomic_cmpxchg(&po->pressure, has_room, !has_room)) {}
>>
>
> This makes no sense to me.
>
> I thought you wanted to avoid dirtying the cache line.
> No atomic op can help the race here.

I principally want to avoid the lock contention on sk_receive_queue.lock,
which is held for a lot longer while probing frames. But yes, I'd prefer to
avoid the cacheline contention as well.

The alternative is to keep the race and just replace the xchg with a
straight assignment.