On 1/8/26 04:23, Jason Wang wrote:

On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
[off-list ref] wrote:

quoted

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 <redacted>
Signed-off-by: Tim Gebauer <redacted>
Signed-off-by: Simon Schippers <redacted>
---
 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.

This sounds like a bug that needs to be fixed, as it requires the user
to know the implementation details. For example, even if
__ptr_ring_consume_created_space() returns true, __ptr_ring_produce()
may still fail?

No, it should not fail in that case.
If you only call consume and after that try to produce, *then* it is
likely to fail because __ptr_ring_zero_tail() is only invoked once per
batch.

Maybe revert fb9de9704775d?

I disagree, as I consider this to be one of the key features of ptr_ring.

That said, there are several other implementation details that users need
to be aware of.

For example, __ptr_ring_empty() must only be called by the consumer. This
was for example the issue in dc82a33297fc ("veth: apply qdisc
backpressure on full ptr_ring to reduce TX drops") and the reason why
5442a9da6978 ("veth: more robust handing of race to avoid txq getting
stuck") exists.

quoted

+ */
+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

Thanks