[RFC PATCH 3/4 v0.3] sched/umcg: RFC: add userspace sll helpers

[Peter Oskolkov <posk@posk.io>]

The helpers below can work with userspace single-linked lists concurrently
without indefinitely spinning in the kernel. Specifically:

push (add to the head of the list):

  step = 0
  while (++step < N)
     old = *head
     *next = old
     cmp_xchng(head, &old, next)

pop (remove the first element from the list):
    mark the node as deleted by flipping its lowest bit without
    actually removing the node from the list:

  curr = *head
  step = 0

  while (curr && ++step < N)

	next = *curr
	if (next & 1)
		curr = next & ~1
		continue

	if (cmp_xchng(curr, next, next | 1))
		return curr
	else
		curr = next & ~1

It is the userspace's responsibility to actually remove the
nodes marked as deleted from the list.

v0.2->v0.3 changes:
  - removed spinning from the kernel (other than capped cmpxchg).

Signed-off-by: Peter Oskolkov <posk@google.com>
---
 kernel/sched/umcg.h | 116 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 116 insertions(+)

diff --git a/kernel/sched/umcg.h b/kernel/sched/umcg.h
index 288017f5368c..435531d751f2 100644
--- a/kernel/sched/umcg.h
+++ b/kernel/sched/umcg.h
@@ -165,5 +165,121 @@ static inline int cmpxchg_user_64(u64 __user *uaddr, u64 *old, u64 new)
 	ret;								\
 })

+/**
+ * umcg_sll_push - push a node onto a single-linked-list (stack) in userspace
+ * @head      - a pointer to the head of the stack;
+ * @node      - a pointer to the node to push;
+ * @max_tries - the maximum number of attempts to push the node onto the list.
+ *
+ * Push a node onto a single-linked list (stack). The function tries
+ * @max_tries times and returns -EAGAIN if too many concurrent updates, so that
+ * the caller may yield the CPU.
+ *
+ * Return:
+ * 0        - success;
+ * -EFAULT  - error;
+ * -EAGAIN  - try again.
+ */
+static inline int umcg_sll_push(u64 __user *head, u64 __user *node, int max_tries)
+{
+	int step;
+
+	for (step = 0; step < max_tries; step++) {
+		int ret;
+		u64 first;
+
+		smp_mb();  /* Make the read below clean. */
+		if (get_user(first, head))
+			return -EFAULT;
+
+		if (put_user(first, node))
+			return -EFAULT;
+		smp_mb();  /* Make the write above visible. */
+
+		ret = cmpxchg_user_64(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;
+	}
+
+	return -EAGAIN;
+}
+
+/**
+ * umcg_sll_pop - pop a node from a single-linked-list (stack) in the userspace
+ * @head      - a pointer to the head of the stack;
+ * @value     - a pointer to where store the popped value;
+ * @max_tries - the maximum number of nodes to try to pop.
+ *
+ * Pop a node from a single-linked list (stack). Internally, popped nodes
+ * are marked as such, and not actually removed from the list, to avoid
+ * the ABA problem. It is the responsibility of the userspace to do GC.
+ *
+ * As an additional protection, the function checks only the first
+ * @max_tries nodes.
+ *
+ * Note: on success, @value should be cast to (u64 __user *) if not zero.
+ *
+ * Return:
+ * 0        - success;
+ * -EFAULT  - error;
+ * -EAGAIN  - try again.
+ */
+static inline int umcg_sll_pop(u64 __user *head, u64 *value, int max_tries)
+{
+	int step;
+	u64 curr;
+
+	smp_mb();  /* Make the read below clean. */
+	if (get_user(curr, head))
+		return -EFAULT;
+
+	for (step = 0; step < max_tries; step++) {
+		int ret;
+		u64 next;
+
+		if (!curr) {
+			*value = 0UL;
+			return 0;
+		}
+
+		if (curr & 1UL)
+			return -EFAULT;  /* Should not happen. */
+
+		smp_mb();  /* Make the read below clean. */
+		if (get_user(next, (u64 __user *)curr))
+			return -EFAULT;
+
+		if (next & 1UL) { /* curr is deleted */
+			curr = next & ~1UL;
+			continue;
+		}
+
+		ret = cmpxchg_user_64((u64 __user *)curr, &next, next | 1UL);
+		if (!ret) {
+			*value = curr;
+			return 0;
+		}
+		if (ret == -EAGAIN) {
+			curr = next & ~1UL;
+			cpu_relax();
+			continue;
+		}
+
+		if (ret)
+			return -EFAULT;
+	}
+
+	return -EAGAIN;
+}
 #endif  /* CONFIG_X86_64 */
 #endif  /* _KERNEL_SCHED_UMCG_H */
--
2.25.1