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

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

[Anthony Iliopoulos]

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(
     [	AC_LANG_PROGRAM([[
 #define _GNU_SOURCE
 #include <urcu.h>
-- 
2.42.0

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

[Darrick J. Wong]

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:

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.

--D

>      [	AC_LANG_PROGRAM([[
>  #define _GNU_SOURCE
>  #include <urcu.h>
> -- 
> 2.42.0
> 

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

[Anthony Iliopoulos]

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

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

[Darrick J. Wong]

On Wed, Sep 06, 2023 at 05:19:52PM +0200, Anthony Iliopoulos wrote:
> 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.

Yeah.  These days 32-bit architectures are supported but not maximally
performant. :/

--D

> 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