1From 439fa2eae78a8900bda120072335be19d626498c Mon Sep 17 00:00:00 2001
2From: "Yann E. MORIN" <yann.morin.1998@free.fr>
3Date: Sun, 28 Dec 2014 23:39:40 +0100
4Subject: [PATCH] erts/ethread: instruct libatomic_ops we do require CAS
5
6We do require compare-and-swap (CAS), so we must instruct libatomic_ops
7to provide it, even if the architecture does not have instructions for
8it.
9
10For example, on ARM, LDREX is required for fast CAS. But LDREX is only
11available on ARMv6, so by default libatomic_ops will not have CAS for
12anything below, like ARMv5. But ARMv5 is always UP, so using an
13emulated CAS (that is signal-asyn-safe) is still possible (albeit much
14slower).
15
16Tell libatomic_ops to provide CAS, even if the hardware is not capable
17of 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
24Thanks go to Thomas for all this insight! :-)
25Thanks go to Frank for reporting the issue! :-)
26
27Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
28Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
29Cc: Frank Hunleth <fhunleth@troodon-software.com>
30---
31 erts/include/internal/libatomic_ops/ethread.h | 1 +
32 1 file changed, 1 insertion(+)
33
34diff --git a/erts/include/internal/libatomic_ops/ethread.h b/erts/include/internal/libatomic_ops/ethread.h
35index 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"
46diff --git a/erts/aclocal.m4 b/erts/aclocal.m4
47index 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--
691.9.1
70
71