[release-branch.go1.11] runtime: avoid arm64 8.1 atomics on Android

The kernel on some Samsung S9+ models reports support for arm64 8.1
atomics, but in reality only some of the cores support them. Go
programs scheduled to cores without support will crash with SIGILL.

This change unconditionally disables the optimization on Android.
A better fix is to precisely detect the offending chipset.

Fixes #28586

Change-Id: I35a1273e5660603824d30ebef2ce7e429241bf1f
Reviewed-on: https://go-review.googlesource.com/c/147377
Run-TryBot: Elias Naur <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/149557
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Elias Naur

src/runtime/os_linux_arm64.go

@@ -29,7 +29,17 @@ func archauxv(tag, val uintptr) {
 		randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
 			uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
 	case _AT_HWCAP:
-		cpu_hwcap = uint(val)
+		// arm64 doesn't have a 'cpuid' instruction equivalent and relies on
+		// HWCAP/HWCAP2 bits for hardware capabilities.
+		hwcap := uint(val)
+		if GOOS == "android" {
+			// The Samsung S9+ kernel reports support for atomics, but not all cores
+			// actually support them, resulting in SIGILL. See issue #28431.
+			// TODO(elias.naur): Only disable the optimization on bad chipsets.
+			const hwcap_ATOMICS = 1 << 8
+			hwcap &= ^uint(hwcap_ATOMICS)
+		}
+		cpu_hwcap = hwcap
 	case _AT_HWCAP2:
 		cpu_hwcap2 = uint(val)
 	}