From: Eric Dumazet <[email protected]>
Date: Thu, 14 May 2015 09:24:46 -0700
> On Thu, 2015-05-14 at 11:53 -0400, Willem de Bruijn wrote:
>
>> 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.
>
> Please describe the race. It seems quite innocent at first look.
>
> Clearly putting xchg() gives a false sense of security in this context.
>
> Atomic ops should be reserved for cases we cannot avoid them,
> not to give false hopes ;)
Basically, ->pressure seems to exist merely to optimize the scanner
in fanout_demux_rollover(). It makes it so that we don't check
sockets we already know lack space.
It is set (in an unlocked context) by packet_rcv_has_room() calls
which calculate that the socket lacks space.
It is cleared either in non-tpacket recvmsg() or poll(), the latter
of which holds the socket receive queue spinlock.
This kind of variable and conditional locking is crummy, at best.
Since non-tpacket recvmsg already has to hold the receive queue lock
to pull out the SKB (via skb_recv_datagram()), there is no value to
the conditional locking done by packet_rcv_has_room().
Just take the receive queue lock always, and then you can guarantee
that all ->pressure updates occur under that lock.
Tests can be done asynchronously without locking in the
fanout_demux_rollover() code, and that's fine. It's a heuristic
after all.
Like this:
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 31d5856..0947895 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1301,17 +1301,14 @@ static int packet_rcv_has_room(struct packet_sock *po,
struct sk_buff *skb)
int ret;
bool has_room;
- if (po->prot_hook.func == tpacket_rcv) {
- spin_lock(&po->sk.sk_receive_queue.lock);
- ret = __packet_rcv_has_room(po, skb);
- spin_unlock(&po->sk.sk_receive_queue.lock);
- } else {
- ret = __packet_rcv_has_room(po, skb);
- }
+ spin_lock(&po->sk.sk_receive_queue.lock);
+ ret = __packet_rcv_has_room(po, skb);
has_room = ret == ROOM_NORMAL;
if (po->pressure == has_room)
- xchg(&po->pressure, !has_room);
+ po->pressure = !has_room;
+
+ spin_unlock(&po->sk.sk_receive_queue.lock);
return ret;
}
@@ -3814,7 +3811,7 @@ static unsigned int packet_poll(struct file *file, struct
socket *sock,
mask |= POLLIN | POLLRDNORM;
}
if (po->pressure && __packet_rcv_has_room(po, NULL) == ROOM_NORMAL)
- xchg(&po->pressure, 0);
+ po->pressure = 0;
spin_unlock_bh(&sk->sk_receive_queue.lock);
spin_lock_bh(&sk->sk_write_queue.lock);
if (po->tx_ring.pg_vec) {
--
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