Re: [Qemu-devel] [PATCH 2/3] net: introduce lock to protect NetClientState's send_queue

[liu ping fan]

On Tue, Mar 5, 2013 at 10:45 AM, liu ping fan <address@hidden> wrote:
> On Mon, Mar 4, 2013 at 10:49 PM, Stefan Hajnoczi <address@hidden> wrote:
>> On Sun, Mar 03, 2013 at 09:21:21PM +0800, Liu Ping Fan wrote:
>>> From: Liu Ping Fan <address@hidden>
>>>
>>> Use nc->transfer_lock to protect the nc->peer->send_queue. All of the
>>
>> Please use consistent names: the lock protects ->send_queue so it's best
>> called send_queue_lock or send_lock.
>>
> OK.
>>> deleter and senders will sync on this lock, so we can also survive across
>>> unplug.
>>>
>>> Signed-off-by: Liu Ping Fan <address@hidden>
>>> ---
>>>  include/net/net.h   |    4 +++
>>>  include/net/queue.h |    1 +
>>>  net/hub.c           |   21 +++++++++++++-
>>>  net/net.c           |   72 
>>> ++++++++++++++++++++++++++++++++++++++++++++++++---
>>>  net/queue.c         |   15 +++++++++-
>>>  5 files changed, 105 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/include/net/net.h b/include/net/net.h
>>> index 24563ef..3e4b9df 100644
>>> --- a/include/net/net.h
>>> +++ b/include/net/net.h
>>> @@ -63,6 +63,8 @@ typedef struct NetClientInfo {
>>>  } NetClientInfo;
>>>
>>>  struct NetClientState {
>>> +    /* protect peer's send_queue */
>>> +    QemuMutex transfer_lock;
>>>      NetClientInfo *info;
>>>      int link_down;
>>>      QTAILQ_ENTRY(NetClientState) next;
>>> @@ -78,6 +80,7 @@ struct NetClientState {
>>>
>>>  typedef struct NICState {
>>>      NetClientState ncs[MAX_QUEUE_NUM];
>>> +    NetClientState *pending_peer[MAX_QUEUE_NUM];
>>
>> Please rebase onto github.com/stefanha/qemu.git net.  ncs[] is no longer
>> statically sized to MAX_QUEUE_NUM.
>>
> OK
>>>      NICConf *conf;
>>>      void *opaque;
>>>      bool peer_deleted;
>>> @@ -105,6 +108,7 @@ NetClientState *qemu_find_vlan_client_by_name(Monitor 
>>> *mon, int vlan_id,
>>>                                                const char *client_str);
>>>  typedef void (*qemu_nic_foreach)(NICState *nic, void *opaque);
>>>  void qemu_foreach_nic(qemu_nic_foreach func, void *opaque);
>>> +int qemu_can_send_packet_nolock(NetClientState *sender);
>>>  int qemu_can_send_packet(NetClientState *nc);
>>>  ssize_t qemu_sendv_packet(NetClientState *nc, const struct iovec *iov,
>>>                            int iovcnt);
>>> diff --git a/include/net/queue.h b/include/net/queue.h
>>> index f60e57f..0ecd23b 100644
>>> --- a/include/net/queue.h
>>> +++ b/include/net/queue.h
>>> @@ -67,6 +67,7 @@ ssize_t qemu_net_queue_send_iov(NetQueue *queue,
>>>                                  NetPacketSent *sent_cb);
>>>
>>>  void qemu_net_queue_purge(NetQueue *queue, NetClientState *from);
>>> +void qemu_net_queue_purge_all(NetQueue *queue);
>>>  bool qemu_net_queue_flush(NetQueue *queue);
>>>
>>>  #endif /* QEMU_NET_QUEUE_H */
>>> diff --git a/net/hub.c b/net/hub.c
>>> index 81d2a04..97c3ac3 100644
>>> --- a/net/hub.c
>>> +++ b/net/hub.c
>>> @@ -53,9 +53,14 @@ static ssize_t net_hub_receive(NetHub *hub, NetHubPort 
>>> *source_port,
>>>          if (port == source_port) {
>>>              continue;
>>>          }
>>> -
>>> +        qemu_mutex_lock(&port->nc.transfer_lock);
>>> +        if (!port->nc.peer) {
>>
>> .peer is protected by transfer_lock too?  This was not documented above
>> and I think it's not necessary to protect .peer?
>>
> The transfer_lock has two aims:
>     to protect the send path against remove path. (lock for nc->peer)
>     to protect among the senders (lock for nc->peer->send_queue)
>>> +            qemu_mutex_unlock(&port->nc.transfer_lock);
>>> +            continue;
>>> +        }
>>>          qemu_net_queue_append(port->nc.peer->send_queue, &port->nc,
>>>                              QEMU_NET_PACKET_FLAG_NONE, buf, len, NULL);
>>> +        qemu_mutex_unlock(&port->nc.transfer_lock);
>>>          event_notifier_set(&port->e);
>>>      }
>>>      return len;
>>> @@ -65,7 +70,13 @@ static void hub_port_deliver_packet(void *opaque)
>>>  {
>>>      NetHubPort *port = (NetHubPort *)opaque;
>>>
>>> +    qemu_mutex_lock(&port->nc.transfer_lock);
>>> +    if (!port->nc.peer) {
>>> +        qemu_mutex_unlock(&port->nc.transfer_lock);
>>> +        return;
>>> +    }
>>>      qemu_net_queue_flush(port->nc.peer->send_queue);
>>> +    qemu_mutex_unlock(&port->nc.transfer_lock);
>>>  }
>>>
>>>  static ssize_t net_hub_receive_iov(NetHub *hub, NetHubPort *source_port,
>>> @@ -78,10 +89,16 @@ static ssize_t net_hub_receive_iov(NetHub *hub, 
>>> NetHubPort *source_port,
>>>          if (port == source_port) {
>>>              continue;
>>>          }
>>> -
>>> +        qemu_mutex_lock(&port->nc.transfer_lock);
>>> +        if (!port->nc.peer) {
>>> +            qemu_mutex_unlock(&port->nc.transfer_lock);
>>> +            continue;
>>> +        }
>>>          qemu_net_queue_append_iov(port->nc.peer->send_queue, &port->nc,
>>>                              QEMU_NET_PACKET_FLAG_NONE, iov, iovcnt, NULL);
>>> +        qemu_mutex_unlock(&port->nc.transfer_lock);
>>>          event_notifier_set(&port->e);
>>> +
>>>      }
>>>      return len;
>>>  }
>>> diff --git a/net/net.c b/net/net.c
>>> index 544542b..0acb933 100644
>>> --- a/net/net.c
>>> +++ b/net/net.c
>>> @@ -207,6 +207,7 @@ static void qemu_net_client_setup(NetClientState *nc,
>>>          nc->peer = peer;
>>>          peer->peer = nc;
>>>      }
>>> +    qemu_mutex_init(&nc->transfer_lock);
>>>      QTAILQ_INSERT_TAIL(&net_clients, nc, next);
>>>
>>>      nc->send_queue = qemu_new_net_queue(nc);
>>> @@ -285,6 +286,7 @@ void *qemu_get_nic_opaque(NetClientState *nc)
>>>
>>>  static void qemu_cleanup_net_client(NetClientState *nc)
>>>  {
>>> +    /* This is the place where may be out of big lock, when dev finalized 
>>> */
>>
>> I don't understand this comment.
>>
> Will remove. I had recorded it to remind myself that extra lock is needed 
> here.
>>>      QTAILQ_REMOVE(&net_clients, nc, next);
>>>
>>>      if (nc->info->cleanup) {
>>> @@ -307,6 +309,28 @@ static void qemu_free_net_client(NetClientState *nc)
>>>      }
>>>  }
>>>
>>> +/* exclude race with rx/tx path, flush out peer's queue */
>>> +static void qemu_flushout_net_client(NetClientState *nc)
>>
>> This function detaches the peer, the name should reflect that.
>>
> OK.
>>> +{
>>> +    NetClientState *peer;
>>> +
>>> +    /* sync on receive path */
>>> +    peer = nc->peer;
>>> +    if (peer) {
>>> +        qemu_mutex_lock(&peer->transfer_lock);
>>> +        peer->peer = NULL;
>>> +        qemu_mutex_unlock(&peer->transfer_lock);
>>> +    }
>>
>> This is weird.  You don't lock to read nc->peer but you do lock to write
>> peer->peer.  If you use a lock it must be used consistently.
> Because removal is the only code path to assign  nc->peer = NULL,  so
> the reader and updater is serial here.  But as for peer->peer, it must
> run against sender.
>
The race between removers is excluded by big lock in hot-unplug path.
>
> Thanks and regards,
> Pingfan