Re: [PATCH] tell gcc optimizer to never introduce new data races

[Richard Biener <richard.guenther@gmail.com>]

On June 10, 2014 8:04:13 PM CEST, Steven Noonan <steven@uplinklabs.net> wrote:
>On Tue, Jun 10, 2014 at 10:46 AM, Linus Torvalds
><torvalds@linux-foundation.org> wrote:
>> On Tue, Jun 10, 2014 at 6:23 AM, Jiri Kosina <jkosina@suse.cz> wrote:
>>> We have been chasing a memory corruption bug, which turned out to be
>>> caused by very old gcc (4.3.4), which happily turned conditional
>load into
>>> a non-conditional one, and that broke correctness (the condition was
>met
>>> only if lock was held) and corrupted memory.
>>
>> Just out of interest, can you point to the particular kernel code
>that
>> caused this? I think that's more interesting than the example program
>> you show - which I'm sure is really nice for gcc developers as an
>> example, but from a kernel standpoint I think it's more important to
>> show the particular problems this caused for the kernel?
>>
>
>Jiri, is there a workaround for compilers that don't support '--param
>allow-store-data-races=0'? For example:

The optimization that purposely performs the undesired transform is loop store motion which is part of the tree loop invariant motion optimization. You can disable that with -fno-tree-loop-im.

That the bug didn't appear with newer compilers was due to lucky decisions to not inline a particular function.

Richard.

>$ gcc-4.5 -O2 -o cond_store cond_store.c && ./cond_store
>Segmentation fault (core dumped)
>
>$ gcc-4.5 -O2 --param allow-store-data-races=0 -o cond_store
>cond_store.c && ./cond_store
>cc1: error: invalid parameter ‘allow-store-data-races’
>
>$ gcc-4.5 -v
>Using built-in specs.
>COLLECT_GCC=gcc-4.5
>COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.5.4/lto-wrapper
>Target: x86_64-unknown-linux-gnu
>Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
>--infodir=/usr/share/info --libdir=/usr/lib --libexecdir=/usr/lib
>--program-suffix=-4.5 --enable-shared
>--enable-languages=c,c++,fortran,objc,obj-c++ --enable-__cxa_atexit
>--disable-libstdcxx-pch --disable-multilib --disable-libgomp
>--disable-libmudflap --disable-libssp --enable-clocale=gnu
>--with-tune=generic --with-cloog --with-ppl --with-system-zlib
>Thread model: posix
>gcc version 4.5.4 (GCC)