Re: [PATCH 2/2] rpc: Add -EPERM processing for xs_udp_send_request()

[Jason Baron <jbaron@akamai.com>]

On 09/19/2014 03:41 PM, Trond Myklebust wrote:
> On Thu, Sep 18, 2014 at 9:54 PM, Jason Baron <jbaron@akamai.com> wrote:
>> On 09/18/2014 05:20 PM, Trond Myklebust wrote:
>>> On Thu, Sep 18, 2014 at 5:02 PM, Jason Baron <jbaron@akamai.com> wrote:
>>>> On 09/18/2014 04:51 PM, Trond Myklebust wrote:
>>>>> On Thu, Sep 18, 2014 at 3:51 PM, Jason Baron <jbaron@akamai.com> wrote:
>>>>>> If an iptables drop rule is added for an nfs server, the client can end up in
>>>>>> a softlockup. Because of the way that xs_sendpages() is structured, the -EPERM
>>>>>> is ignored since the prior bits of the packet may have been successfully queued
>>>>>> and thus xs_sendpages() returns a non-zero value. Then, xs_udp_send_request()
>>>>>> thinks that because some bits were queued it should return -EAGAIN. We then try
>>>>>> the request and again and a softlockup occurs. The test sequence is simply:
>>>>>>
>>>>>> 1) open a file on the nfs server '/nfs/foo' (mounted using udp)
>>>>>> 2) iptables -A OUTPUT -d <nfs server ip> -j DROP
>>>>>> 3) write to /nfs/foo
>>>>>> 4) close /nfs/foo
>>>>>> 5) iptables -D OUTPUT -d <nfs server ip> -j DROP
>>>>>>
>>>>>> The softlockup occurs in step 4 above.
>>>>> For UDP, the expected and documented behaviour in the case above is as follows:
>>>>> - if the mount is soft, then return EIO on the first major timeout.
>>>> yeah - so this case is a softlockup in my testing :(
>>>>
>>>>> - if the mount is hard, then retry indefinitely on timeout.
>>>>>
>>>>> Won't these 2 patches end up propagating an EPERM to the application?
>>>>> That would be a definite violation of both hard and soft semantics.
>>>> ok, yeah it does propogate the -EPERM up - I wasn't aware of the correct
>>>> semantics - thanks.
>>>>
>>>> I can rework the patches such that they return -EIO instead for a soft mount,
>>>> and verify that we keep retrying for a hard one.
>>>>
>>> Doesn't the soft timeout currently trigger after the major timeout? If
>>> not, do we understand why it isn't doing so?
>>
>> No, the soft timeout does not currently trigger after the major timeout. Instead,
>> the kernel spins indefinitely, and triggers a softlockup.
>>
>> The reason is that xs_sendpages() returns a positive value in this case
>> and xs_udp_send_request() turns it in an -EAGAIN for the write operation.
>> Subsequently, we call call_transmit_status() and then call_status() which
>> sees the EAGAIN, which just starts all over again with a 'call_transmit()'.
>> So we are stuck spinning indefinitely in kernel space.
>>
>> Simply moving the -EPERM up in this patch, results in the behavior you
>> described above - EIO after a major timeout on a soft mount, and indefinte
>> retries on a hard mount - but without the cpu consumption. IE applying
>> this on top of this patch:
>>
>> --- a/net/sunrpc/clnt.c
>> +++ b/net/sunrpc/clnt.c
>> @@ -2019,6 +2019,7 @@ call_status(struct rpc_task *task)
>>         case -EHOSTDOWN:
>>         case -EHOSTUNREACH:
>>         case -ENETUNREACH:
>> +       case -EPERM:
>>                 if (RPC_IS_SOFTCONN(task)) {
>>                         rpc_exit(task, status);
>>                         break;
>> @@ -2048,7 +2049,6 @@ call_status(struct rpc_task *task)
>>         case -EAGAIN:
>>                 task->tk_action = call_transmit;
>>                 break;
>> -       case -EPERM:
>>         case -EIO:
>>                 /* shutdown or soft timeout */
>>                 rpc_exit(task, status);
>>
>> We could also 'translate' the -EPERM into an '-ENETUNREACH' or such,
>> in the return from xs_udp_send_request(), if you think that would make
>> more sense?
>>
>> Hopefully, I've explained things better.
>>
>>
> 
> Yep. Can you please resend the patch with the above fix? I think we
> can live with the EPERM in the RPC_IS_SOFTCONN case, given that it is
> in practice only ever passed back to the 'mount' syscall.
> 

Hi,

So after some more testing on this new patch, the test sequence I described
works fine - but if I set the firewall rule first and then do an open, it
appears that the open() wouldn't time out even on a soft mount (whereas
with the previous patch it incorrectly returned -EPERM almost immediately).
It appears that the rpc request is getting queued up onto one of the wait
queues (xprt_backlog or xprt_sending) in that case, but I'm not sure why.
I'll have to look more into it next week.

Thanks,

-Jason