On Fri, Jan 09, 2026 at 02:01:54PM +0800, Jason Wang wrote:

On Thu, Jan 8, 2026 at 3:21 PM Simon Schippers
[off-list ref] wrote:

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

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

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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.

Well, this makes the helper very hard for users.

So I think at least the documentation should specify the meaning of
'n' here.

Right. Documenting parameters is good.

For example, is it the value returned by
ptr_ring_consume_batched()(), and is it required to be called
immediately after ptr_ring_consume_batched()? If it is, the API is
kind of tricky to be used, we should consider to merge two helpers
into a new single helper to ease the user.

I think you are right partially it's more a question of documentation and naming.
It's not that it's hard to use: follow up patches use it
without issues - it's that neither documentatin nor
naming explain how.

let's try to document, first of all: if it does not guarantee that
produce will succeed, then what *is* the guarantee this API gives?

What's more, there would be false positives. Considering there's not
many entries in the ring, just after the first zeroing,
__ptr_ring_consume_created_space() will return true, this will lead to
unnecessary wakeups.

well optimizations are judged on their performance not on theoretical
analysis. in this instance, this should be rare enough.

And last, the function will always succeed if n is greater than the batch.

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Maybe revert fb9de9704775d?

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

Nope, it's just an optimization and actually it changes the behaviour
that might be noticed by the user.

Before the patch, ptr_ring_produce() is guaranteed to succeed after a
ptr_ring_consume(). After that patch, it's not. We don't see complaint
because the implication is not obvious (e.g more packet dropping).

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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.

At least the behaviour is documented. This is not the case for the
implications of fb9de9704775d.

Thanks

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+ */
+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