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[packages/trusty/cirros-testvm.git] / cirros-testvm / src-cirros / buildroot-2015.05 / 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