Re: [RFC PATCH 2/3 v0.2] sched/umcg: RFC: add userspace atomic helpers

+static inline int umcg_atomic_cmpxchg_64(u64 *uval, u64 __user *uaddr,
+						u64 oldval, u64 newval)
+{
+	int ret = 0;
+
+	if (!user_access_begin(uaddr, sizeof(u64)))
+		return -EFAULT;
+	asm volatile("\n"
+		"1:\t" LOCK_PREFIX "cmpxchgq %4, %2\n"
+		"2:\n"
+		"\t.section .fixup, \"ax\"\n"
+		"3:\tmov     %3, %0\n"
+		"\tjmp     2b\n"
+		"\t.previous\n"
+		_ASM_EXTABLE_UA(1b, 3b)
+		: "+r" (ret), "=a" (oldval), "+m" (*uaddr)
+		: "i" (-EFAULT), "r" (newval), "1" (oldval)
+		: "memory"
+	);
+	user_access_end();
+	*uval = oldval;
+	return ret;
+}

+static inline int fix_pagefault(unsigned long uaddr, bool write_fault)
+{
+	struct mm_struct *mm = current->mm;
+	int ret;
+
+	mmap_read_lock(mm);
+	ret = fixup_user_fault(mm, uaddr, write_fault ? FAULT_FLAG_WRITE : 0,
+			NULL);
+	mmap_read_unlock(mm);
+
+	return ret < 0 ? ret : 0;
+}

+static inline int umcg_cmpxchg_64_user(u64 __user *uaddr, u64 *prev, u64 val)
+{
+	while (true) {
+		int ret;
+		u64 expected = *prev;
+
+		pagefault_disable();
+		ret = umcg_atomic_cmpxchg_64(prev, uaddr, expected, val);
+		pagefault_enable();
+
+		if (!ret)
+			return *prev == expected ? 0 : -EAGAIN;
+
+		if (WARN_ONCE(ret != -EFAULT, "Unexpected error"))
+			return -EFAULT;
+
+		ret = fix_pagefault((unsigned long)uaddr, true);
+		if (ret)
+			return -EFAULT;
+	}
+}
+
+/**
+ * atomic_stack_push_user - push a node onto the stack
+ * @head - a pointer to the head of the stack;
+ * @node - a pointer to the node to push.
+ *
+ * Push a node onto a single-linked list (stack). Atomicity/correctness
+ * is guaranteed by locking the head via settings its first bit (assuming
+ * the pointer is aligned).
+ *
+ * Return: 0 on success, -EFAULT on error.
+ */
+static inline int atomic_stack_push_user(u64 __user *head, u64 __user *node)
+{
+	while (true) {
+		int ret;
+		u64 first;
+
+		smp_mb();  /* Make the read below clean. */
+		if (get_user(first, head))
+			return -EFAULT;
+
+		if (first & 1UL) {
+			cpu_relax();
+			continue;  /* first is being deleted. */
+		}
+
+		if (put_user(first, node))
+			return -EFAULT;
+		smp_mb();  /* Make the write above visible. */
+
+		ret = umcg_cmpxchg_64_user(head, &first, (u64)node);
+		if (!ret)
+			return 0;
+
+		if (ret == -EAGAIN) {
+			cpu_relax();
+			continue;
+		}
+
+		if (WARN_ONCE(ret != -EFAULT, "unexpected umcg_cmpxchg result"))
+			return -EFAULT;
+
+		return -EFAULT;
+	}
+}