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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 5 Dec 2018 14:41:30 +0100
Message-ID: <CAMuHMdU9zXXSaPHKvfG3A73h3CTsb9H2RT_gWt-Ne=qQ+HKShQ@mail.gmail.com>
Subject: Re: NFS/TCP crashes on MIPS/RBTX4927 in v4.20-rcX (bisected)
To:     Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc:     Trond Myklebust <trond.myklebust@hammerspace.com>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@vger.kernel.org>,
        linux-mips@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Nemoto-san,

On Wed, Dec 5, 2018 at 2:11 PM Atsushi Nemoto <anemo@mba.ocn.ne.jp> wrote:
> On Tue, 4 Dec 2018 14:53:07 +0100, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > I found similar crashes in a report from 2006, but of course the code
> > has changed too much to apply the solution proposed there
> > (https://www.linux-mips.org/archives/linux-mips/2006-09/msg00169.html).
> >
> > Userland is Debian 8 (the last release supporting "old" MIPS).
> > My kernel is based on v4.20.0-rc5, but the issue happens with v4.20-rc1,
> > too.
> >
> > However, I noticed it works in v4.19! Hence I've bisected this, to commit
> > 277e4ab7d530bf28 ("SUNRPC: Simplify TCP receive code by switching to using
> > iterators").
> >
> > Dropping the ",tcp" part from the nfsroot parameter also fixes the issue.
> >
> > Given RBTX4927 is little endian, just like my arm/arm64 boards, it's probably
> > not an endianness issue.  Sparse didn't show anything suspicious before/after
> > the guilty commit.
> >
> > Do you have a clue?
>
> If it was a cache issue, disabling i-cache or d-cache completely might
> help understanding the problem.  I added TXx9 specific "icdisable" and
> "dcdisable" kernel options for debugging long ago.
>
> I hope these options still works correctly with recent kernel but not
> sure.
>
> Also, disabling i-cache makes your board VERY slow, of course.

Thanks!

When using these options, I do see a slowdown in early boot, but the issue
is still there.

My next guess is an unaligned access not using {get,put}_unaligned(), which
doesn't seem to work on tx4927, but doesn't cause an exception neither.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds