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

[Steven Noonan <steven@uplinklabs.net>]

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:

$ 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)