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Date:   Fri, 8 Dec 2017 12:26:46 -0800
From:   Matt Turner <mattst88@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        linux-nfs@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Manuel Lauss <manuel.lauss@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: NFS corruption, fixed by echo 1 > /proc/sys/vm/drop_caches --
 next debugging steps?

On Fri, Dec 8, 2017 at 5:52 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2017-12-08 at 05:42 -0800, Eric Dumazet wrote:
>> On Thu, Dec 7, 2017 at 11:54 PM, Matt Turner <mattst88@...il.com>
>> wrote:
>> > On Thu, Dec 7, 2017 at 11:00 PM, Matt Turner <mattst88@...il.com>
>> > wrote:
>> > > On Sun, Mar 12, 2017 at 6:43 PM, Matt Turner <mattst88@...il.com>
>> > > wrote:
>> > > > On a Broadcom BCM91250a MIPS system I can reliably trigger NFS
>> > > > corruption on the first file read.
>> > > >
>> > > > To demonstrate, I downloaded five identical copies of the gcc-
>> > > > 5.4.0
>> > > > source tarball. On the NFS server, they hash to the same value:
>> > > >
>> > > > server distfiles # md5sum gcc-5.4.0.tar.bz2*
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.1
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.2
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.3
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.4
>> > > >
>> > > > On the MIPS system (the NFS client):
>> > > >
>> > > > bcm91250a-le distfiles # md5sum gcc-5.4.0.tar.bz2.2
>> > > > 35346975989954df8a8db2b034da610d  gcc-5.4.0.tar.bz2.2
>> > > > bcm91250a-le distfiles # md5sum gcc-5.4.0.tar.bz2*
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.1
>> > > > 35346975989954df8a8db2b034da610d  gcc-5.4.0.tar.bz2.2
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.3
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.4
>> > > >
>> > > > The first file read will contain some corruption, and it is
>> > > > persistent until...
>> > > >
>> > > > bcm91250a-le distfiles # echo 1 > /proc/sys/vm/drop_caches
>> > > > bcm91250a-le distfiles # md5sum gcc-5.4.0.tar.bz2*
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.1
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.2
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.3
>> > > > 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.4
>> > > >
>> > > > the caches are dropped, at which point it reads back properly.
>> > > >
>> > > > Note that the corruption is different across reboots, both in
>> > > > the size
>> > > > of the corruption and the location. I saw 1900~ and 1400~ byte
>> > > > sequences corrupted on separate occasions, which don't
>> > > > correspond to
>> > > > the system's 16kB page size.
>> > > >
>> > > > I've tested kernels from v3.19 to 4.11-rc1+ (master branch from
>> > > > today). All exhibit this behavior with differing frequencies.
>> > > > Earlier
>> > > > kernels seem to reproduce the issue less often, while more
>> > > > recent
>> > > > kernels reliably exhibit the problem every boot.
>> > > >
>> > > > How can I further debug this?
>> > >
>> > > I think I was wrong about the statement about kernels v3.19 to
>> > > 4.11-rc1+. I found out I couldn't reproduce with 4.7-rc1 and then
>> > > bisected to 4cd13c21b207e80ddb1144c576500098f2d5f882 ("softirq:
>> > > Let
>> > > ksoftirqd do its job"). Still reproduces with current tip of
>> > > Linus'
>> > > tree.
>> > >
>> > > Any ideas? The board's ethernet is an uncommon device supported
>> > > by
>> > > CONFIG_SB1250_MAC. Something about the ethernet driver maybe?
>> >
>> > With the patch reverted on master (reverts cleanly), NFS corruption
>> > no
>> > longer happens.
>>
>> Hi Matt.
>>
>> Thanks for bisecting.
>>
>> Patch simply exposes an existing bug more often by changing the way
>> driver functions are scheduled.
>>
>> Which is probably a good thing.
>>
>> sbmac_intr() looks extremely suspicious to me.
>>
>> A NAPI driver hard interrupt should simply schedule NAPI.
>>
>> Apparently, if sbmac_intr() can not grab NAPIF_STATE_SCHED bit, it
>> directly calls sbdma_rx_process() from
>> hard interrupt context.
>>
>> Insane really.
>
> Please try this fix (not compiled on my x86 laptop, and I had no coffee
> yet, so it might have some trivial errors)

Thanks for the quick reply!

I tried the patch on top of master, but unfortunately the corruption
still occurs.

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