ptr_ring: add ptr_ring_unconsume

Applications that consume a batch of entries in one go
can benefit from ability to return some of them back
into the ring.

Add an API for that - assuming there's space. If there's no space
naturally can't do this and have to drop entries, but this implies ring
is full so we'd likely drop some anyway.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 6b2e0dd88569b13c66ef445609b2f12419fdd3ac..796b90f6d4e90028783a7b085456f48f1150ae21 100644 (file)
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -403,6 +403,61 @@ static inline int ptr_ring_init(struct ptr_ring *r, int size, gfp_t gfp)
        return 0;
 }
 
+/*
+ * Return entries into ring. Destroy entries that don't fit.
+ *
+ * Note: this is expected to be a rare slow path operation.
+ *
+ * Note: producer lock is nested within consumer lock, so if you
+ * resize you must make sure all uses nest correctly.
+ * In particular if you consume ring in interrupt or BH context, you must
+ * disable interrupts/BH when doing so.
+ */
+static inline void ptr_ring_unconsume(struct ptr_ring *r, void **batch, int n,
+                                     void (*destroy)(void *))
+{
+       unsigned long flags;
+       int head;
+
+       spin_lock_irqsave(&r->consumer_lock, flags);
+       spin_lock(&r->producer_lock);
+
+       if (!r->size)
+               goto done;
+
+       /*
+        * Clean out buffered entries (for simplicity). This way following code
+        * can test entries for NULL and if not assume they are valid.
+        */
+       head = r->consumer_head - 1;
+       while (likely(head >= r->consumer_tail))
+               r->queue[head--] = NULL;
+       r->consumer_tail = r->consumer_head;
+
+       /*
+        * Go over entries in batch, start moving head back and copy entries.
+        * Stop when we run into previously unconsumed entries.
+        */
+       while (n) {
+               head = r->consumer_head - 1;
+               if (head < 0)
+                       head = r->size - 1;
+               if (r->queue[head]) {
+                       /* This batch entry will have to be destroyed. */
+                       goto done;
+               }
+               r->queue[head] = batch[--n];
+               r->consumer_tail = r->consumer_head = head;
+       }
+
+done:
+       /* Destroy all entries left in the batch. */
+       while (n)
+               destroy(batch[--n]);
+       spin_unlock(&r->producer_lock);
+       spin_unlock_irqrestore(&r->consumer_lock, flags);
+}
+
 static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
                                           int size, gfp_t gfp,
                                           void (*destroy)(void *))