netdev - [PATCH 1/4] ptr_ring_spare: Helper to check if spare capacity of size cnt is available

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Message-ID: <20250902080957.47265-2-simon.schippers@tu-dortmund.de>
Date: Tue,  2 Sep 2025 10:09:54 +0200
From: Simon Schippers <simon.schippers@...dortmund.de>
To: willemdebruijn.kernel@...il.com, jasowang@...hat.com, mst@...hat.com,
        eperezma@...hat.com, stephen@...workplumber.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux.dev, kvm@...r.kernel.org
Cc: Simon Schippers <simon.schippers@...dortmund.de>,
        Tim Gebauer <tim.gebauer@...dortmund.de>
Subject: [PATCH 1/4] ptr_ring_spare: Helper to check if spare capacity of size cnt is available

The implementation is inspired by ptr_ring_empty.

Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
---
 include/linux/ptr_ring.h | 71 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 551329220e4f..6b8cfaecf478 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -243,6 +243,77 @@ static inline bool ptr_ring_empty_bh(struct ptr_ring *r)
 	return ret;
 }
 
+/*
+ * Check if a spare capacity of cnt is available without taking any locks.
+ *
+ * If cnt==0 or cnt > r->size it acts the same as __ptr_ring_empty.
+ *
+ * The same requirements apply as described for __ptr_ring_empty.
+ */
+static inline bool __ptr_ring_spare(struct ptr_ring *r, int cnt)
+{
+	int size = r->size;
+	int to_check;
+
+	if (unlikely(!size || cnt < 0))
+		return true;
+
+	if (cnt > size)
+		cnt = 0;
+
+	to_check = READ_ONCE(r->consumer_head) - cnt;
+
+	if (to_check < 0)
+		to_check += size;
+
+	return !r->queue[to_check];
+}
+
+static inline bool ptr_ring_spare(struct ptr_ring *r, int cnt)
+{
+	bool ret;
+
+	spin_lock(&r->consumer_lock);
+	ret = __ptr_ring_spare(r, cnt);
+	spin_unlock(&r->consumer_lock);
+
+	return ret;
+}
+
+static inline bool ptr_ring_spare_irq(struct ptr_ring *r, int cnt)
+{
+	bool ret;
+
+	spin_lock_irq(&r->consumer_lock);
+	ret = __ptr_ring_spare(r, cnt);
+	spin_unlock_irq(&r->consumer_lock);
+
+	return ret;
+}
+
+static inline bool ptr_ring_spare_any(struct ptr_ring *r, int cnt)
+{
+	unsigned long flags;
+	bool ret;
+
+	spin_lock_irqsave(&r->consumer_lock, flags);
+	ret = __ptr_ring_spare(r, cnt);
+	spin_unlock_irqrestore(&r->consumer_lock, flags);
+
+	return ret;
+}
+
+static inline bool ptr_ring_spare_bh(struct ptr_ring *r, int cnt)
+{
+	bool ret;
+
+	spin_lock_bh(&r->consumer_lock);
+	ret = __ptr_ring_spare(r, cnt);
+	spin_unlock_bh(&r->consumer_lock);
+
+	return ret;
+}
+
 /* Must only be called after __ptr_ring_peek returned !NULL */
 static inline void __ptr_ring_discard_one(struct ptr_ring *r)
 {
-- 
2.43.0