dccp ccid-2: Implementation of circular Ack Vector buffer with overflow handling

This completes the implementation of a circular buffer for Ack Vectors, by
extending the current (linear array-based) implementation.  The changes are:

 (a) An `overflow' flag to deal with the case of overflow. As before, dynamic
     growth of the buffer will not be supported; but code will be added to deal
     robustly with overflowing Ack Vector buffers.

 (b) A `tail_seqno' field. When naively implementing the algorithm of Appendix A
     in RFC 4340, problems arise whenever subsequent Ack Vector records overlap,
     which can bring the entire run length calculation completely out of synch.
     (This is documented on http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/\
                                             ack_vectors/tracking_tail_ackno/ .)
 (c) The buffer length is now computed dynamically (i.e. current fill level),
     as the span between head to tail.

As a result, dccp_ackvec_pending() is now simpler - the #ifdef is no longer
necessary since buf_empty is always true when IP_DCCP_ACKVEC is not configured.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

1 files changed, 30 insertions, 1 deletions

diff --git a/net/dccp/ackvec.c b/net/dccp/ackvec.c
index af976fca407a..abaf241c7353 100644
--- a/net/dccp/ackvec.c
+++ b/net/dccp/ackvec.c
@@ -29,7 +29,7 @@ struct dccp_ackvec *dccp_ackvec_alloc(const gfp_t priority)
         struct dccp_ackvec *av = kmem_cache_zalloc(dccp_ackvec_slab, priority);
 
         if (av != NULL) {
-                av->av_buf_head = DCCPAV_MAX_ACKVEC_LEN - 1;
+                av->av_buf_head = av->av_buf_tail = DCCPAV_MAX_ACKVEC_LEN - 1;
                 INIT_LIST_HEAD(&av->av_records);
         }
         return av;
@@ -72,6 +72,14 @@ int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum)
         avr->avr_ack_nonce  = nonce_sum;
         avr->avr_ack_runlen = dccp_ackvec_runlen(av->av_buf + av->av_buf_head);
         /*
+         * When the buffer overflows, we keep no more than one record. This is
+         * the simplest way of disambiguating sender-Acks dating from before the
+         * overflow from sender-Acks which refer to after the overflow; a simple
+         * solution is preferable here since we are handling an exception.
+         */
+        if (av->av_overflow)
+                dccp_ackvec_purge_records(av);
+        /*
          * Since GSS is incremented for each packet, the list is automatically
          * arranged in descending order of @ack_seqno.
          */
@@ -85,6 +93,27 @@ int dccp_ackvec_update_records(struct dccp_ackvec *av, u64 seqno, u8 nonce_sum)
 }
 
 /*
+ * Buffer index and length computation using modulo-buffersize arithmetic.
+ * Note that, as pointers move from right to left, head is `before' tail.
+ */
+static inline u16 __ackvec_idx_add(const u16 a, const u16 b)
+{
+        return (a + b) % DCCPAV_MAX_ACKVEC_LEN;
+}
+
+static inline u16 __ackvec_idx_sub(const u16 a, const u16 b)
+{
+        return __ackvec_idx_add(a, DCCPAV_MAX_ACKVEC_LEN - b);
+}
+
+u16 dccp_ackvec_buflen(const struct dccp_ackvec *av)
+{
+        if (unlikely(av->av_overflow))
+                return DCCPAV_MAX_ACKVEC_LEN;
+        return __ackvec_idx_sub(av->av_buf_tail, av->av_buf_head);
+}
+
+/*
  * If several packets are missing, the HC-Receiver may prefer to enter multiple
  * bytes with run length 0, rather than a single byte with a larger run length;
  * this simplifies table updates if one of the missing packets arrives.