Re: [Qemu-devel] [RFC PATCH v2 2/4] net: resolve race of tap backend and its peer

[Stefan Hajnoczi]

On Fri, Mar 29, 2013 at 8:21 AM, liu ping fan <address@hidden> wrote:
> On Thu, Mar 28, 2013 at 10:34 PM, Stefan Hajnoczi <address@hidden> wrote:
>> On Thu, Mar 28, 2013 at 03:55:53PM +0800, Liu Ping Fan wrote:
>>> From: Liu Ping Fan <address@hidden>
>>>
>>> Signed-off-by: Liu Ping Fan <address@hidden>
>>> ---
>>>  net/tap.c |    7 ++++++-
>>>  1 files changed, 6 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/net/tap.c b/net/tap.c
>>> index 0b663d1..401161c 100644
>>> --- a/net/tap.c
>>> +++ b/net/tap.c
>>> @@ -321,9 +321,14 @@ static void tap_cleanup(NetClientState *nc)
>>>  static void tap_poll(NetClientState *nc, bool enable)
>>>  {
>>>      TAPState *s = DO_UPCAST(TAPState, nc, nc);
>>> -    /* fixme, when tap backend on another thread, the disable should be 
>>> sync */
>>> +
>>>      tap_read_poll(s, enable);
>>>      tap_write_poll(s, enable);
>>> +
>>> +    if (!enable) {
>>> +        /* need sync so vhost can take over polling */
>>> +        g_source_remove_poll(&nc->nsrc->source, &nc->nsrc->gfd);
>>> +    }
>>
>> Is this necessary?
>>
>> Since we're in tap_poll() we are currently not in poll(2) waiting on the
>> fd.  Therefore it's safe for the caller to use the fd, the prepare
>> function will remove it from the fd set before glib calls poll(2) again.
>>
> The main purpose is to make sure that the GSource->dispatch is not
> in-flight, in which case, both vhost handler and ->dispatch() will
> work on it.

Okay, I see.  hw/vhost_net.c will give the fd to the vhost kernel
module.  We need to be sure the fd is no longer in use - and we don't
want tap readable/writable handlers to be called.

Stefan