[PATCH net-next v7 2/9] ptr_ring: add helper to detect newly freed space on consume

[Simon Schippers]

This proposed function checks whether __ptr_ring_zero_tail() was invoked
within the last n calls to __ptr_ring_consume(), which indicates that new
free space was created. Since __ptr_ring_zero_tail() moves the tail to
the head - and no other function modifies either the head or the tail,
aside from the wrap-around case described below - detecting such a
movement is sufficient to detect the invocation of
__ptr_ring_zero_tail().

The implementation detects this movement by checking whether the tail is
at most n positions behind the head. If this condition holds, the shift
of the tail to its current position must have occurred within the last n
calls to __ptr_ring_consume(), indicating that __ptr_ring_zero_tail() was
invoked and that new free space was created.

This logic also correctly handles the wrap-around case in which
__ptr_ring_zero_tail() is invoked and the head and the tail are reset
to 0. Since this reset likewise moves the tail to the head, the same
detection logic applies.

Co-developed-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
Signed-off-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
---
 include/linux/ptr_ring.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index a5a3fa4916d3..7cdae6d1d400 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -438,6 +438,19 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
 	return ret;
 }
 
+/* Returns true if the consume of the last n elements has created space
+ * in the ring buffer (i.e., a new element can be produced).
+ *
+ * Note: Because of batching, a successful call to __ptr_ring_consume() /
+ * __ptr_ring_consume_batched() does not guarantee that the next call to
+ * __ptr_ring_produce() will succeed.
+ */
+static inline bool __ptr_ring_consume_created_space(struct ptr_ring *r,
+						    int n)
+{
+	return r->consumer_head - r->consumer_tail < n;
+}
+
 /* Cast to structure type and call a function without discarding from FIFO.
  * Function must return a value.
  * Callers must take consumer_lock.
-- 
2.43.0