Add futex support to LKL.

This uses the GCC builtins for the atomic ops, avoiding
architecture-specific code.

arch/lkl/include/asm/Kbuild

@@ -21,7 +21,6 @@ generic-y += errno.h
 generic-y += extable.h
 generic-y += exec.h
 generic-y += ftrace.h
-generic-y += futex.h
 generic-y += hardirq.h
 generic-y += hw_irq.h
 generic-y += ioctl.h

arch/lkl/include/asm/futex.h

@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_LKL_FUTEX_H
+#define _ASM_LKL_FUTEX_H
+
+#ifdef __KERNEL__
+
+#include <linux/futex.h>
+#include <asm/errno.h>
+
+static inline int arch_futex_atomic_op_inuser(int op, int oparg, int *oval,
+		u32 __user *uaddr)
+{
+	int oldval = 0, ret = 0;
+
+	switch (op) {
+	case FUTEX_OP_SET:
+		__atomic_exchange(uaddr, &oparg, oval, __ATOMIC_SEQ_CST);
+		break;
+	case FUTEX_OP_ADD:
+		__atomic_fetch_add(uaddr, oparg, __ATOMIC_SEQ_CST);
+		break;
+	case FUTEX_OP_OR:
+		__atomic_fetch_or(uaddr, oparg, __ATOMIC_SEQ_CST);
+		break;
+	case FUTEX_OP_ANDN:
+		__atomic_fetch_and(uaddr, ~oparg, __ATOMIC_SEQ_CST);
+		break;
+	case FUTEX_OP_XOR:
+		__atomic_fetch_xor(uaddr, oparg, __ATOMIC_SEQ_CST);
+		break;
+	default:
+		ret = -ENOSYS;
+	}
+
+	if (!ret)
+		*oval = oldval;
+
+	return ret;
+}
+
+static inline int futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
+						u32 oldval, u32 newval)
+{
+	if (uaddr == NULL) {
+		return -EFAULT;
+	} else {
+		u32 v = oldval;
+		int ret = __atomic_compare_exchange_n(uaddr, &v, newval, 0 /*weak*/, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
+
+		*uval = v;
+
+		return ret;
+	}
+}
+
+#endif
+#endif