Re: Linux NFSv4.1 client session seqid sometimes advances by 2

[bfields@fieldses.org (J. Bruce Fields)]

On Tue, Apr 13, 2021 at 09:31:37AM -0400, Olga Kornievskaia wrote:
> On Tue, Apr 13, 2021 at 3:08 AM Rick Macklem <rmacklem@uoguelph.ca> wrote:
> >
> > Hi,
> >
> > During testing of a Fedora Core 30 (5.2.10 kernel) against a FreeBSD
> > server (4.1 mount), I have been simulating a network partitioning
> > for a few minutes (until the TCP connection goes to SYN_SENT on
> > the Linux client).
> >
> > Sometimes, after the network partition heals, the FreeBSD server
> > replies NFS4ERR_SEQ_MISORDERED.
> > Looking at the packet trace, the seqid for the slot has advanced by
> > 2 instead of 1. An RPC request for old-seqid + 1 never seems to get
> > sent.
> > --> Since sending an RPC with "seqid + 2" but never sending one
> >        that is "seqid + 1" for a slot seems harmless, I have added an optional
> >        hack (can be turned off), to allow this case instead of replying
> >        NFS4ERR_SEQ_MISORDERED for it. The code will still reply
> >        NFS4ERR_SEQ_MISORDERED if an RPC for the slot with
> >        "old seqid + 1" in it.
> >        --> Yes, doing this hack is a violation of RFC5661, but I've
> >              done it anyhow.
> >
> > If you are interested in a packet capture with this in it:
> > fetch https://people.freebsd.org/~rmacklem/linuxtofreenfs.pcap
> > - then look at packet #1945 and #2072
> >   --> You'll see that slot #1 seqid goes from 4 to 6. There is no
> >          slot#1 seqid 5 RPC sent on the wire.
> >          (This packet capture was taken on the Linux client using
> >           tcpdump.)
> > --> Btw, the "RST battle" you'll see in the above trace between
> >        #2005 and #2068 that goes on until the FreeBSD
> >        krpc/NFS times out the connection after 6min. seems to be a recent
> >        FreeBSD TCP bug.
> >        I have reproduced this seqid advances by 2 on an older system
> >        that does not "RST battle" and allows the reconnect right away,
> >        once the network partition is healed, so it does seem to be
> >        relevant to this bug.
> >
> > Someday, I will get around to upgrading to a more recent Linux
> > kernel and will test to see if I can still reproduce this bug.
> > On 5.2.10, it is intermittent and does not occur every time I
> > do the network partitioning test.
> >
> > Mostly just fyi, rick
> 
> Hi Rick,
> 
> I think this is happening because slotid=1 had something queued up
> using seqid=5 and that was interrupted because the connection was
> RSTed. For the interrupted slot, the client would send solo SEQUENCE
> with +1 seqid.

Doesn't the client send the solo SEQUENCE with seqid 5 in that case?

--b.