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Date:   Tue, 13 Apr 2021 15:29:08 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Olga Kornievskaia <aglo@umich.edu>
Cc:     Rick Macklem <rmacklem@uoguelph.ca>,
        Linux-NFS <linux-nfs@vger.kernel.org>
Subject: Re: Linux NFSv4.1 client session seqid sometimes advances by 2
Message-ID: <20210413192908.GD28230@fieldses.org>
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On Tue, Apr 13, 2021 at 02:59:27PM -0400, Olga Kornievskaia wrote:
> On Tue, Apr 13, 2021 at 1:17 PM J. Bruce Fields <bfields@fieldses.org> wrote:
> >
> > 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?
> 
> No it sends with seq+1 because NFS layer client doesn't know if seqid
> actually was actually transmitted before the connection got caught
> (and/or received by the server).

But then the MISORDERED tells the client it wasn't received, so the
client follows up with a call with seqid 5--is that what happens?

Sorry, I seem to recall we went through this all a couple years ago, but
now I've forgotten how it works.

--b.