| I notice there is other functionality here also...
|
| > +/**
| > + * tfrc_tx_hist_when - Retrieve send time of past packet
| > + * If successful, it garbage-collects older (irrelevant) entries and returns 1.
| > + */
| > +int tfrc_tx_hist_when(ktime_t *stamp, struct tfrc_tx_hist_head *head, u64 ackno)
| > +{
| > +
| > + if (tail)
| > + __tfrc_tx_hist_remove_tail(tail, head->cache);
| > +
|
| Can you explain the rationale behind this?
|
You have cut out half of the code, I therefore need to copy again below:
+int tfrc_tx_hist_when(ktime_t *stamp, struct tfrc_tx_hist_head *head, u64 ackno)
+{
+ struct tfrc_tx_hist *cur, *tail = NULL;
+
+ write_lock_bh(&tfrc_tx_hist_lock);
+ for (cur = head->first; cur != NULL; cur = cur->next)
+ if (cur->seqno == ackno) {
+ *stamp = cur->stamp;
+ tail = cur->next;
+ cur->next = NULL;
+ break;
+ }
+ write_unlock_bh(&tfrc_tx_hist_lock);
+
+ if (tail)
+ __tfrc_tx_hist_remove_tail(tail, head->cache);
+
+ return (cur != NULL);
+}
The patch first performs a lookup. If the lookup is successful, tail != NULL and
all entries older than `cur' (LIFO order) are cut off; at the same time tail is set
to the `tail' starting at the now old/obsolete entries.
Removing the tail is not done when under lock, since this may take time (you never
know how long the list is). So this uses a common trick of first `orphaning' the
old tail, and then doing the cleanup outside the lock-protected region, when there
is more time to do so.
Maybe you have noted that the subsequent patch (6/6) has the following hunk:
@@ -492,9 +488,6 @@ static void ccid3_hc_tx_packet_recv(stru
/* unschedule no feedback timer */
sk_stop_timer(sk, &hctx->ccid3hctx_no_feedback_timer);
- /* remove all packets older than the one acked from history */
- dccp_tx_hist_purge_older(ccid3_tx_hist,
- &hctx->ccid3hctx_hist, packet);
/*
* As we have calculated new ipi, delta, t_nom it is possible
* that we now can send a packet, so wake up dccp_wait_for_ccid
=> The above effectively replaces this in one go. This is better since otherwise
the same lock would need to be acquired twice - once for reading, then for
writing (to perform the purge). This would introduce a race condition.
=> You can find this also in the documentation of patch 5/6:
+ * tfrc_tx_hist_when - Retrieve send time of past packet
+
+ * If successful, it garbage-collects older (irrelevant) entries and returns 1.
+ */
-
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