[Gluster-devel] Possible bug in the communications layer ?

Xavier Hernandez xhernandez at datalab.es
Fri Apr 29 07:51:57 UTC 2016


Hi Raghavendra,

yes, the readv response contains xdata. The dict length is 38 (0x26) 
and, at the moment of failure, rsp.xdata.xdata_len already contains 0x26.

Xavi

On 29/04/16 09:10, Raghavendra Gowdappa wrote:
>
>
> ----- Original Message -----
>> From: "Raghavendra Gowdappa" <rgowdapp at redhat.com>
>> To: "Xavier Hernandez" <xhernandez at datalab.es>
>> Cc: "Gluster Devel" <gluster-devel at gluster.org>
>> Sent: Friday, April 29, 2016 12:36:43 PM
>> Subject: Re: [Gluster-devel] Possible bug in the communications layer ?
>>
>>
>>
>> ----- Original Message -----
>>> From: "Raghavendra Gowdappa" <rgowdapp at redhat.com>
>>> To: "Xavier Hernandez" <xhernandez at datalab.es>
>>> Cc: "Jeff Darcy" <jdarcy at redhat.com>, "Gluster Devel"
>>> <gluster-devel at gluster.org>
>>> Sent: Friday, April 29, 2016 12:07:59 PM
>>> Subject: Re: [Gluster-devel] Possible bug in the communications layer ?
>>>
>>>
>>>
>>> ----- Original Message -----
>>>> From: "Xavier Hernandez" <xhernandez at datalab.es>
>>>> To: "Jeff Darcy" <jdarcy at redhat.com>
>>>> Cc: "Gluster Devel" <gluster-devel at gluster.org>
>>>> Sent: Thursday, April 28, 2016 8:15:36 PM
>>>> Subject: Re: [Gluster-devel] Possible bug in the communications layer ?
>>>>
>>>>
>>>>
>>>> Hi Jeff,
>>>>
>>>> On 28.04.2016 15:20, Jeff Darcy wrote:
>>>>
>>>>
>>>>
>>>> This happens with Gluster 3.7.11 accessed through Ganesha and gfapi. The
>>>> volume is a distributed-disperse 4*(4+2). I'm able to reproduce the
>>>> problem
>>>> easily doing the following test: iozone -t2 -s10g -r1024k -i0 -w
>>>> -F/iozone{1..2}.dat echo 3 >/proc/sys/vm/drop_caches iozone -t2 -s10g
>>>> -r1024k -i1 -w -F/iozone{1..2}.dat The error happens soon after starting
>>>> the
>>>> read test. As can be seen in the data below, client3_3_readv_cbk() is
>>>> processing an iovec of 116 bytes, however it should be of 154 bytes (the
>>>> buffer in memory really seems to contain 154 bytes). The data on the
>>>> network
>>>> seems ok (at least I haven't been able to identify any problem), so this
>>>> must be a processing error on the client side. The last field in cut
>>>> buffer
>>>> of the sequentialized data corresponds to the length of the xdata field:
>>>> 0x26. So at least 38 more byte should be present.
>>>> Nice detective work, Xavi.  It would be *very* interesting to see what
>>>> the value of the "count" parameter is (it's unfortunately optimized out).
>>>> I'll bet it's two, and iov[1].iov_len is 38.  I have a weak memory of
>>>> some problems with how this iov is put together, a couple of years ago,
>>>> and it looks like you might have tripped over one more.
>>>> It seems you are right. The count is 2 and the first 38 bytes of the
>>>> second
>>>> vector contains the remaining data of xdata field.
>>>
>>> This is the bug. client3_3_readv_cbk (and for that matter all the
>>> actors/cbks) expects response in utmost two vectors:
>>> 1. Program header containing request or response. This is subjected to
>>> decoding/encoding. This vector should point to a buffer that contains the
>>> entire program header/response contiguously.
>>> 2. If the procedure returns payload (like readv response or a write
>>> request),
>>> second vector contains the buffer pointing to the entire (contiguous)
>>> payload. Note that this payload is raw and is not subjected to
>>> encoding/decoding.
>>>
>>> In your case, this _clean_ separation is broken with part of program header
>>> slipping into 2nd vector supposed to contain read data (may be because of
>>> rpc fragmentation). I think this is a bug in socket layer. I'll update more
>>> on this.
>>
>> Does your read response include xdata too? I think the code related to
>> reading xdata in readv response is a bit murky.
>>
>> <socket.c/__socket_read_accepted_successful_reply>
>>
>>         case SP_STATE_ACCEPTED_SUCCESS_REPLY_INIT:
>>                 default_read_size = xdr_sizeof ((xdrproc_t)
>>                 xdr_gfs3_read_rsp,
>>                                                 &read_rsp);
>>
>>                 proghdr_buf = frag->fragcurrent;
>>
>>                 __socket_proto_init_pending (priv, default_read_size);
>>
>>                 frag->call_body.reply.accepted_success_state
>>                         = SP_STATE_READING_PROC_HEADER;
>>
>>                 /* fall through */
>>
>>         case SP_STATE_READING_PROC_HEADER:
>>                 __socket_proto_read (priv, ret);
>>
>>>>>>> By this time we've read read response _minus_ the xdata
>
> I meant we have read "readv response header"
>
>>
>>                 /* there can be 'xdata' in read response, figure it out */
>>                 xdrmem_create (&xdr, proghdr_buf, default_read_size,
>>                                XDR_DECODE);
>>
>>>>>>>> We created xdr stream above with "default_read_size" (this doesn't
>>>>>>>> include xdata)
>>
>>                 /* This will fail if there is xdata sent from server, if not,
>>                    well and good, we don't need to worry about  */
>>
>>>>>>>> what if xdata is present and decoding failed (as length of xdr stream
>>>>>>>> above - default_read_size - doesn't include xdata)? would we have a
>>>>>>>> valid value in read_rsp.xdata.xdata_len? This is the part I am
>>>>>>>> confused about. If read_rsp.xdata.xdata_len is not correct then there
>>>>>>>> is a possibility that xdata might not be entirely present in the
>>>>>>>> vector socket passes to higher layers as progheader (with part or
>>>>>>>> entire xdata spilling over to payload vector).
>>
>>                 xdr_gfs3_read_rsp (&xdr, &read_rsp);
>>
>>                 free (read_rsp.xdata.xdata_val);
>>
>>                 /* need to round off to proper roof (%4), as XDR packing pads
>>                    the end of opaque object with '0' */
>>                 size = roof (read_rsp.xdata.xdata_len, 4);
>>
>>                 if (!size) {
>>                         frag->call_body.reply.accepted_success_state
>>                                 = SP_STATE_READ_PROC_OPAQUE;
>>                         goto read_proc_opaque;
>>                 }
>>
>> </socket.c>
>>
>> Can you please confirm whether there was an xdata in the readv response (may
>> be by looking in bricks) whose decoding failed?
>>
>> regards,
>> Raghavendra
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>> Gluster-devel at gluster.org
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>>


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