[Gluster-devel] Possible bug in the communications layer ?
Raghavendra Gowdappa
rgowdapp at redhat.com
Fri Apr 29 07:10:27 UTC 2016
----- 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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