Re: [PATCH 3/3] SUNRPC: svc_xprt_enqueue should not refuse to enqueue 'XPT_DEAD' transports

Tom Tucker
2008-12-23T23:39:18Z

Tom Tucker wrote:
> Trond Myklebust wrote:
>> On Wed, 2008-12-17 at 09:35 -0600, Tom Tucker wrote:
>>
>>> Trond Myklebust wrote:
>>>
>>>> Aside from being racy (there is nothing preventing someone setting
>>>> XPT_DEAD
>>>> after the test in svc_xprt_enqueue, and before XPT_BUSY is set), it is
>>>> wrong to assume that transports which have called svc_delete_xprt()
>>>> might
>>>> not need to be re-enqueued.
>>>>
>>> This is only true because now you allow transports with XPT_DEAD set
>>> to be enqueued -- yes?
>>>
>>>
>>>> See the list of deferred requests, which is currently never going to
>>>> be cleared if the revisit call happens after svc_delete_xprt(). In
>>>> this
>>>> case, the deferred request will currently keep a reference to the
>>>> transport
>>>> forever.
>>>>
>>>>
>>> I agree this is a possibility and it needs to be fixed. I'm
>>> concerned that the root cause is still there though. I thought the
>>> test case was the client side timing out the connection. Why are
>>> there deferred requests sitting on what is presumably an idle
>>> connection?
>>>
>>
>> I haven't said that they are the cause of this test case. I've said that
>> deferred requests hold references to the socket that can obviously
>> deadlock. That needs to be fixed regardless of whether or not it is the
>> cause here.
>>
>> There are plenty of situations in which the client may choose to close
>> the TCP socket even if there are outstanding requests. One of the most
>> common is when the user signals the process, so that an RPC call that
>> was partially transmitted (ran out of buffer space) gets cancelled
>> before it can finish transmitting. In that case the client has no choice
>> but to disconnect and immediately reconnect.
>>
>>
>>>> The fix should be to allow dead transports to be enqueued in order
>>>> to clear
>>>> the deferred requests, then change the order of processing in
>>>> svc_recv() so
>>>> that we pick up deferred requests before we do the XPT_CLOSE
>>>> processing.
>>>>
>>>>
>>> Wouldn't it be simpler to clean up any deferred requests in the
>>> close path instead of changing the meaning of XPT_DEAD and
>>> dispatching N-threads to do the same?
>>>
>>
>> AFAICS, deferred requests are the property of the cache until they
>> expire or a downcall occurs. I'm not aware of any way to cancel only
>> those deferred requests that hold a reference to this particular
>> transport.
>>
>>
> Ok, I think you're right, and I think that this fix is correct and
> makes the symptom go away.
>
> I may be completely confused here, but:
>
> - The deferred requests should be getting cleaned up by timing out,
> and that does not not seem to be happening, (Is this true?)
>
They are getting "cleaned up", but by the time they do the transport is
dead, the request has been added to the deferred queue, but it won't get
processed because svc_xprt_enqueue won't "schedule" a dead transport.

> - By releasing the underlying connection prior to releasing the
> transport that manages it, we've converted the visible resource leek
> to an invisible one.
>
Not with your changes per the above.

> - This has been around forever and changing the client side close
> behavior graceful exposed this bug,
>
> So I'm wondering if what we want to do here is to provide a mechanism
> for canceling deferred requests for a particular transport. This would
> provide a mechanism for the generic transport driver to force
> cancellation of deferred requests when closing.
This is a new interface and we'd still need to handle requests sitting
on the transport's deferred queue. Probably not a good idea.

> Tom
>
>
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