| 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 |
| |
