linux-kernel - Re: nios2 crash/hang in mainline due to 'lib: update LZ4 compressor module'

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Date:   Wed, 1 Mar 2017 11:45:21 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>
Cc:     Sandra Loosemore <sandra@...esourcery.com>,
        Tobias Klauser <tklauser@...tanz.ch>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Ley Foon Tan <lftan@...era.com>,
        nios2-dev@...ts.rocketboards.org
Subject: Re: nios2 crash/hang in mainline due to 'lib: update LZ4 compressor
 module'

On Wed, Mar 01, 2017 at 07:58:17PM +0100, Sven Schmidt wrote:
> Hi Guenter, Tobias and Sandra,
> 
> thanks for your effort here.
> 
> On Tue, Feb 28, 2017 at 10:14:13AM -0800, Guenter Roeck wrote:
> > On Tue, Feb 28, 2017 at 10:53:56AM -0700, Sandra Loosemore wrote:
> > > On 02/28/2017 08:53 AM, Tobias Klauser wrote:
> > > >(adding Sandra Loosemore to Cc due to possible relation to gcc/binutils
> > > >for nios2)
> > > >
> > > >On 2017-02-26 at 22:03:38 +0100, Guenter Roeck <linux@...ck-us.net> wrote:
> > > >>Hi Sven,
> > > >>
> > > >>my qemu test for nios2 started failing with commit 4e1a33b105dd ("lib:
> > > >>update LZ4 compressor module"). The test hangs early during boot before
> > > >>any console output is seen. Reverting the offending patch as well as the
> > > >>subsequent lz4 related patches fixes the problem. Disabling CONFIG_RD_LZ4
> > > >>and with it other LZ4 options also fixes it (as does adding "return -EINVAL;"
> > > >>at the top of the LZ4 decompression code). For reference, bisect log
> > > >>is attached.
> > > >>
> > > >>I tried with buildroot toolchains using gcc 6.1.0 as well as 6.3.0
> > > >>and binutils 2.26.1. Scripts used to run the tests are available at
> > > >>https://github.com/groeck/linux-build-test/tree/master/rootfs/nios2.
> > > >>Qemu is from qemu mainline or qemu v2.8 with nios2 patches applied.
> > > >
> > > >Looks like this is somehow related to gcc/binutils. Using GCC 4.8.3 and
> > > >binutils 2.24.51 (both from from Sourcery CodeBench Lite 2014.05) I can
> > > >get a kernel booting on latest master branch. AFAICT, none of the
> > > >LZ4_decompress_* functions are called during boot.
> > > >
> 
> It seems a bit strange that code which is not actually called causes problems like that.
> 
Yes, it is, though it is always possible. The code isn't exactly easy to
understand; there may be some hidden caveats such as global variables. It may
also be that some jump target exceeds its range (though why that would only
be seen with the LZ4 code is another question), or that the compiler gets
confused by the forced inlines (disabling that didn't make a difference,
though, nor did disabling -O3).

> Please let me know if and how I may help you figure out what's happening, especially
> regarding the differences between the previous LZ4 and the current implementation.
> 

For my part I am all but clueless. Unless someone has an idea, we may to
disable LZ4 support for nios2 for the time being. Does anyone have thoughts
on that ? Of course, that would not help if the problem also affects
recent gcc/binutil versions on other architectures.

Thanks,
Guenter

> > > >However, using a self-built GCC 7.0 (20161127) and binutils 2.27 I can
> > > >reproduce the problem you see using the instructions Guenter provided in
> > > >the reply to Sven.
> > > >
> > > >I'll try to dig a bit deeper from here on. Any suggestions on what to
> > > >look out for wrt the differences between the gcc/binutils version are
> > > >welcome of course.
> > > 
> > > This message doesn't give me enough context to know what is going on,
> > > especially without seeing the rest of the thread.  Generally speaking,
> > > Mentor recommends you use one of our stable releases instead of trying to
> > > roll your own from mainline sources.  As an upstream binutils and gcc
> > > maintainer I do try my best to look at bug reports for those components, but
> > > I need a reproducible standalone testcase and specific versions of the
> > > different components involved.
> > > 
> > The problem is also seen with Sourcery CodeBench Lite 2016.11-32 (gcc 6.2.0,
> > binutils 2.26.51). I can provide additional details if needed, but we don't
> > have a well enough understanding of the problem to be able to provide a
> > reduced size test case. The test used to reproduce the problem is available
> > at https://github.com/groeck/linux-build-test/tree/master/rootfs/nios2,
> > run on the ToT linux kernel.
> > 
> > Guenter
> 
> Regards,
> 
> Sven