4a9df29424
For some platforms, hardware-assisted compare-and-swap may not be
available, so libatomic_ops will not provide it.
However, libatomic_ops can provide a purely software CAS emulation, but
must be instructed to do so. erlang just forgot to tell libatomic_ops
that it does require CAS.
Fix that by defining AO_REQUIRE_CAS before including atmoic_ops.h, like
is done in libunwind, as pointed out by Thomas.
Also, erlang has a convoluted, mind-alterating set on aclocal.m4 macros,
that just forgets to link against -latomic_ops when checking CAS is
available, so that even if CAS is available, configure chokes.
Since I would like to keep the little sanity I still have, just force
linking with -latomic_ops. This is useless when the check is natrally
sucessful (i.e. on platforms where CAS is available in HW), but we
would eventually link with -latomic_ops there, too; it's just redundant.
Overall, just consider that erlang requires libatomic_ops, so forcibly
depend on it, it is easier than trying to disable it. We can revisit
that whenever someone wants to run erlang on a platform for which there
is no libatomic_ops support.
Fixes a slew of autobuild ARM failures:
http://autobuild.buildroot.org/results/e7b/e7bfc4893dea6b133f0794ef44d50ad89bcb6662/
http://autobuild.buildroot.org/results/3e9/3e9c307f1ec6536482641019dcaa94677f7267a3/
http://autobuild.buildroot.org/results/a85/a85ca414e5b67af46510abd7b610eb5ae8661de4/
[...]
[Thomas: fix minor typos in commit log, add dependency on
BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS to the Erlang comment about thread
and shared library dependency.]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
71 lines
2.5 KiB
Diff
71 lines
2.5 KiB
Diff
From 439fa2eae78a8900bda120072335be19d626498c Mon Sep 17 00:00:00 2001
|
|
From: "Yann E. MORIN" <yann.morin.1998@free.fr>
|
|
Date: Sun, 28 Dec 2014 23:39:40 +0100
|
|
Subject: [PATCH] erts/ethread: instruct libatomic_ops we do require CAS
|
|
|
|
We do require compare-and-swap (CAS), so we must instruct libatomic_ops
|
|
to provide it, even if the architecture does not have instructions for
|
|
it.
|
|
|
|
For example, on ARM, LDREX is required for fast CAS. But LDREX is only
|
|
available on ARMv6, so by default libatomic_ops will not have CAS for
|
|
anything below, like ARMv5. But ARMv5 is always UP, so using an
|
|
emulated CAS (that is signal-asyn-safe) is still possible (albeit much
|
|
slower).
|
|
|
|
Tell libatomic_ops to provide CAS, even if the hardware is not capable
|
|
of it, by using emulated CAS, as per libatomic_ops dosc:
|
|
https://github.com/ivmai/libatomic_ops/blob/master/doc/README.txt#L28
|
|
|
|
If this is included after defining AO_REQUIRE_CAS, then the package
|
|
will make an attempt to emulate compare-and-swap in a way that (at
|
|
least on Linux) should still be async-signal-safe.
|
|
|
|
Thanks go to Thomas for all this insight! :-)
|
|
Thanks go to Frank for reporting the issue! :-)
|
|
|
|
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
|
|
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
|
|
Cc: Frank Hunleth <fhunleth@troodon-software.com>
|
|
---
|
|
erts/include/internal/libatomic_ops/ethread.h | 1 +
|
|
1 file changed, 1 insertion(+)
|
|
|
|
diff --git a/erts/include/internal/libatomic_ops/ethread.h b/erts/include/internal/libatomic_ops/ethread.h
|
|
index d65ee19..71d3598 100644
|
|
--- a/erts/include/internal/libatomic_ops/ethread.h
|
|
+++ b/erts/include/internal/libatomic_ops/ethread.h
|
|
@@ -35,6 +35,7 @@
|
|
|
|
#define ETHR_NATIVE_IMPL__ "libatomic_ops"
|
|
|
|
+#define AO_REQUIRE_CAS
|
|
#include "atomic_ops.h"
|
|
#include "ethr_membar.h"
|
|
#include "ethr_atomic.h"
|
|
diff --git a/erts/aclocal.m4 b/erts/aclocal.m4
|
|
index d65ee19..71d3598 100644
|
|
--- a/erts/aclocal.m4
|
|
+++ b/erts/aclocal.m4
|
|
@@ -1414,7 +1414,8 @@
|
|
fi;;
|
|
esac
|
|
ethr_have_libatomic_ops=no
|
|
- AC_TRY_LINK([#include "atomic_ops.h"],
|
|
+ AC_TRY_LINK([#define AO_REQUIRE_CAS
|
|
+ #include "atomic_ops.h"],
|
|
[
|
|
volatile AO_t x;
|
|
AO_t y;
|
|
@@ -1455,6 +1455,7 @@
|
|
AC_CHECK_SIZEOF(AO_t, ,
|
|
[
|
|
#include <stdio.h>
|
|
+ #define AO_REQUIRE_CAS
|
|
#include "atomic_ops.h"
|
|
])
|
|
AC_DEFINE_UNQUOTED(ETHR_SIZEOF_AO_T, $ac_cv_sizeof_AO_t, [Define to the size of AO_t if libatomic_ops is used])
|
|
--
|
|
1.9.1
|
|
|