runtime, sync/atomic: fix arm cas
Works around bug in kernel implementation on old ARM5 kernels.
Bug was fixed on 26 Nov 2007 (between 2.6.23 and 2.6.24) but
old kernels persist.
Fixes #1750.
R=dfc, golang-dev
CC=golang-dev
https://golang.org/cl/4436072
diff --git a/src/pkg/runtime/linux/arm/sys.s b/src/pkg/runtime/linux/arm/sys.s
index d866b0e..2b5365b 100644
--- a/src/pkg/runtime/linux/arm/sys.s
+++ b/src/pkg/runtime/linux/arm/sys.s
@@ -258,11 +258,22 @@
TEXT runtime·cas(SB),7,$0
MOVW valptr+0(FP), R2
MOVW old+4(FP), R0
+casagain:
MOVW new+8(FP), R1
BL cas<>(SB)
- MOVW $0, R0
- MOVW.CS $1, R0
+ BCC cascheck
+ MOVW $1, R0
RET
+cascheck:
+ // Kernel lies; double-check.
+ MOVW valptr+0(FP), R2
+ MOVW old+4(FP), R0
+ MOVW 0(R2), R3
+ CMP R0, R3
+ BEQ casagain
+ MOVW $0, R0
+ RET
+
TEXT runtime·casp(SB),7,$0
B runtime·cas(SB)
