package/erlang/0002-erts-ethread-instruct-libatomic_ops-we-do-require-CA.patch

   1 From 439fa2eae78a8900bda120072335be19d626498c Mon Sep 17 00:00:00 2001
   2 From: "Yann E. MORIN" <yann.morin.1998@free.fr>
   3 Date: Sun, 28 Dec 2014 23:39:40 +0100
   4 Subject: [PATCH] erts/ethread: instruct libatomic_ops we do require CAS
   5
   6 We do require compare-and-swap (CAS), so we must instruct libatomic_ops
   7 to provide it, even if the architecture does not have instructions for
   8 it.
   9
  10 For example, on ARM, LDREX is required for fast CAS. But LDREX is only
  11 available on ARMv6, so by default libatomic_ops will not have CAS for
  12 anything below, like ARMv5. But ARMv5 is always UP, so using an
  13 emulated CAS (that is signal-asyn-safe) is still possible (albeit much
  14 slower).
  15
  16 Tell libatomic_ops to provide CAS, even if the hardware is not capable
  17 of it, by using emulated CAS, as per libatomic_ops dosc:
  18     https://github.com/ivmai/libatomic_ops/blob/master/doc/README.txt#L28
  19
  20     If this is included after defining AO_REQUIRE_CAS, then the package
  21     will make an attempt to emulate compare-and-swap in a way that (at
  22     least on Linux) should still be async-signal-safe.
  23
  24 Thanks go to Thomas for all this insight! :-)
  25 Thanks go to Frank for reporting the issue! :-)
  26
  27 Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
  28 Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
  29 Cc: Frank Hunleth <fhunleth@troodon-software.com>
  30 ---
  31  erts/include/internal/libatomic_ops/ethread.h | 1 +
  32  1 file changed, 1 insertion(+)
  33
  34 diff --git a/erts/include/internal/libatomic_ops/ethread.h b/erts/include/internal/libatomic_ops/ethread.h
  35 index d65ee19..71d3598 100644
  36 --- a/erts/include/internal/libatomic_ops/ethread.h
  37 +++ b/erts/include/internal/libatomic_ops/ethread.h
  38 @@ -35,6 +35,7 @@
  39
  40  #define ETHR_NATIVE_IMPL__ "libatomic_ops"
  41
  42 +#define AO_REQUIRE_CAS
  43  #include "atomic_ops.h"
  44  #include "ethr_membar.h"
  45  #include "ethr_atomic.h"
  46 diff --git a/erts/aclocal.m4 b/erts/aclocal.m4
  47 index d65ee19..71d3598 100644
  48 --- a/erts/aclocal.m4
  49 +++ b/erts/aclocal.m4
  50 @@ -1414,7 +1414,8 @@
  51                     fi;;
  52             esac
  53             ethr_have_libatomic_ops=no
  54 -           AC_TRY_LINK([#include "atomic_ops.h"],
  55 +           AC_TRY_LINK([#define AO_REQUIRE_CAS
  56 +                    #include "atomic_ops.h"],
  57                         [
  58                             volatile AO_t x;
  59                             AO_t y;
  60 @@ -1455,6 +1455,7 @@
  61                 AC_CHECK_SIZEOF(AO_t, ,
  62                                 [
  63                                     #include <stdio.h>
  64 +                                   #define AO_REQUIRE_CAS
  65                                     #include "atomic_ops.h"
  66                                 ])
  67                 AC_DEFINE_UNQUOTED(ETHR_SIZEOF_AO_T, $ac_cv_sizeof_AO_t, [Define to the size of AO_t if libatomic_ops is used])
  68 --
  69 1.9.1
  70