Re: [PATCH] libxfs: fix atomic64_t detection on x86 32-bit architectures

[Anthony Iliopoulos <ailiop@suse.com>]

On Tue, Sep 05, 2023 at 09:42:50AM -0700, Darrick J. Wong wrote:
> On Tue, Sep 05, 2023 at 10:46:23AM +0200, Anthony Iliopoulos wrote:
> > xfsprogs during compilation tries to detect if liburcu supports atomic
> > 64-bit ops on the platform it is being compiled on, and if not it falls
> > back to using pthread mutex locks.
> > 
> > The detection logic for that fallback relies on _uatomic_link_error()
> > which is a link-time trick used by liburcu that will cause compilation
> > errors on archs that lack the required support. That only works for the
> > generic liburcu code though, and it is not implemented for the
> > x86-specific code.
> > 
> > In practice this means that when xfsprogs is compiled on 32-bit x86
> > archs will successfully link to liburcu for atomic ops, but liburcu does
> > not support atomic64_t on those archs. It indicates this during runtime
> > by generating an illegal instruction that aborts execution, and thus
> > causes various xfsprogs utils to be segfaulting.
> > 
> > Fix this by executing the liburcu atomic64_t detection code during
> > configure instead of only relying on the linker error, so that
> > compilation will properly fall back to pthread mutexes on those archs.
> > 
> > Fixes: 7448af588a2e ("libxfs: fix atomic64_t poorly for 32-bit architectures")
> > 
> > Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
> > ---
> >  m4/package_urcu.m4 | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/m4/package_urcu.m4 b/m4/package_urcu.m4
> > index ef116e0cda76..f26494a69718 100644
> > --- a/m4/package_urcu.m4
> > +++ b/m4/package_urcu.m4
> > @@ -26,11 +26,15 @@ rcu_init();
> >  #
> >  # Make sure that calling uatomic_inc on a 64-bit integer doesn't cause a link
> >  # error on _uatomic_link_error, which is how liburcu signals that it doesn't
> > -# support atomic operations on 64-bit data types.
> > +# support atomic operations on 64-bit data types for its generic
> > +# implementation (which relies on compiler builtins). For certain archs
> > +# where liburcu carries its own implementation (such as x86_32), it
> > +# signals lack of support during runtime by emitting an illegal
> > +# instruction, so we also need to execute here to detect that.
> >  #
> >  AC_DEFUN([AC_HAVE_LIBURCU_ATOMIC64],
> >    [ AC_MSG_CHECKING([for atomic64_t support in liburcu])
> > -    AC_LINK_IFELSE(
> > +    AC_RUN_IFELSE(
> 
> Unfortunately, this change breaks cross compiling:

Of course.. I completely forgot about that.

> checking for umode_t... no
> checking for atomic64_t support in liburcu... configure: error: in
> 	`.../xfsprogs/build-aarch64':
> configure: error: cannot run test program while cross compiling
> See `config.log' for more details
> 
> (Note that this is an x64 host building aarch64)
> 
> Seeing as we /do/ have a (slow) workaround for 32-bit machines, perhaps
> we should use it any time a long isn't 64-bits wide:
> 
> diff --git a/m4/package_urcu.m4 b/m4/package_urcu.m4
> index ef116e0cda7..2ad4179aca2 100644
> --- a/m4/package_urcu.m4
> +++ b/m4/package_urcu.m4
> @@ -34,8 +34,11 @@ AC_DEFUN([AC_HAVE_LIBURCU_ATOMIC64],
>      [  AC_LANG_PROGRAM([[
>  #define _GNU_SOURCE
>  #include <urcu.h>
> +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>         ]], [[
>  long long f = 3;
> +
> +BUILD_BUG_ON(CAA_BITS_PER_LONG < 64);
>  uatomic_inc(&f);
>         ]])
>      ], have_liburcu_atomic64=yes
> 
> This will cause suboptimal performance on any 32-bit cpu that /does/
> support atomic operations on a u64, but oh well.

I am not sure there is atomic u64 liburcu support for any 32-bit cpu
(even if that cpu does actually support it). Everything is fenced behind
the same conditional (#if CAA_BITS_PER_LONG == 64) in urcu headers
already (e.g. ppc.h or pretty much anything else that falls back to
uatomic/generic.h). So your patch may be the best way forward.

Honestly I am not sure why this isn't implemented at least for x86 (e.g.
via cmpxchg8b). There's a configure option enable-compiler-atomic-builtins
that makes this work, but it doesn't seem to be enabled in distros
(looks fairly new, liburcu commit 3afcf5a0407c).

Regards,
Anthony