[PATCH net-next v14 4/5] ptr_ring: move free-space check into separate helper

[Simon Schippers <simon.schippers@tu-dortmund.de>]

This patch moves the check for available free space for a new entry into
a separate function. Existing callers that only check for a non-zero
return value are unaffected. __ptr_ring_produce() now returns -EINVAL
for a zero-size ring and -ENOSPC when full, whereas before both cases
returned -ENOSPC. The new helper allows callers to determine in advance
whether a single subsequent __ptr_ring_produce() call will succeed. This
information can, for example, be used to temporarily stop producing until
__ptr_ring_check_produce() indicates that space is available again.

The return values are documented above the helper, as a caller that waits
for space must distinguish the transient -ENOSPC from the permanent
-EINVAL.

Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
---
 include/linux/ptr_ring.h | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index d2c3629bbe45..631c43fde440 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -96,6 +96,26 @@ static inline bool ptr_ring_full_bh(struct ptr_ring *r)
 	return ret;
 }
 
+/* Report whether the next __ptr_ring_produce() has room for one entry:
+ * 0 means the single slot at r->queue[r->producer] is free, -ENOSPC means
+ * the ring is full, which is transient, and -EINVAL means r->size is 0,
+ * which is permanent. A caller that stops producing and waits for space
+ * must therefore do so only for -ENOSPC.
+ *
+ * Note: callers invoking this in a loop must use a compiler barrier,
+ * for example cpu_relax(). Callers must hold producer_lock.
+ */
+static inline int __ptr_ring_check_produce(struct ptr_ring *r)
+{
+	if (unlikely(!r->size))
+		return -EINVAL;
+
+	if (data_race(r->queue[r->producer]))
+		return -ENOSPC;
+
+	return 0;
+}
+
 /* Note: callers invoking this in a loop must use a compiler barrier,
  * for example cpu_relax(). Callers must hold producer_lock.
  * Callers are responsible for making sure pointer that is being queued
@@ -103,8 +123,10 @@ static inline bool ptr_ring_full_bh(struct ptr_ring *r)
  */
 static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
 {
-	if (unlikely(!r->size) || data_race(r->queue[r->producer]))
-		return -ENOSPC;
+	int ret = __ptr_ring_check_produce(r);
+
+	if (ret)
+		return ret;
 
 	/* Make sure the pointer we are storing points to a valid data. */
 	/* Pairs with the dependency ordering in __ptr_ring_consume. */
-- 
2.43.0