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 d866b0e220934ecc5de38fcbf7c17eac3074e717..2b5365bd86475b09270d54f3277b9aaadcc50dae 100644 (file)
--- a/src/pkg/runtime/linux/arm/sys.s
+++ b/src/pkg/runtime/linux/arm/sys.s
@@ -258,11 +258,22 @@ TEXT cas<>(SB),7,$0
 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)

diff --git a/src/pkg/sync/atomic/asm_linux_arm.s b/src/pkg/sync/atomic/asm_linux_arm.s
index 5e7aea292e72277d7527671d446aefb7446f8163..72f8d746bb269e2aba73eac3fe35f4f34c3624e4 100644 (file)
--- a/src/pkg/sync/atomic/asm_linux_arm.s
+++ b/src/pkg/sync/atomic/asm_linux_arm.s
@@ -13,6 +13,12 @@
 //     LR = return address
 // The function returns with CS true if the swap happened.
 // http://lxr.linux.no/linux+v2.6.37.2/arch/arm/kernel/entry-armv.S#L850
+// On older kernels (before 2.6.24) the function can incorrectly
+// report a conflict, so we have to double-check the compare ourselves
+// and retry if necessary.
+//
+// http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b49c0f24cf6744a3f4fd09289fe7cade349dead5
+//
 TEXT cas<>(SB),7,$0
        MOVW    $0xffff0fc0, PC
 
@@ -23,12 +29,23 @@ TEXT ·CompareAndSwapInt32(SB),7,$0
 TEXT ·CompareAndSwapUint32(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
+casret:
        MOVW    R0, ret+12(FP)
        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
+       B       casret
 
 TEXT ·CompareAndSwapUintptr(SB),7,$0
        B       ·CompareAndSwapUint32(SB)