tcp_collapse() detects skb to start collapsing from in the
first loop.

If the last skb has SYN or FIN, the condition is tested
again after the loop.

skb_frags_readable() is placed inside and outside of the
loop as well.

If we goto skip_this in both cases, we can remove the second
tests and simply replace end_of_skbs with the inverted loop
condition.

Then, we do not need a test for tcp_collapse_one() too.

Also, skb_frags_readable() should be tested after the fully
overlapping check.

Let's clean up the loop.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv4/tcp_input.c | 28 ++++++++++------------------
 1 file changed, 10 insertions(+), 18 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a2074a1af422..32e63d089db8 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5839,23 +5839,17 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
 {
 	struct sk_buff *skb = head, *n;
 	struct sk_buff_head tmp;
-	bool end_of_skbs;
 
 	/* First, check that queue is collapsible and find
 	 * the point where collapsing can be useful.
 	 */
 restart:
-	for (end_of_skbs = true; skb != NULL && skb != tail; skb = n) {
+	for (; skb && skb != tail; skb = n) {
 		n = tcp_skb_next(skb, list);
 
-		if (!skb_frags_readable(skb))
-			goto skip_this;
-
 		/* No new bits? It is possible on ofo queue. */
 		if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
 			skb = tcp_collapse_one(sk, skb, list, root);
-			if (!skb)
-				break;
 			goto restart;
 		}
 
@@ -5864,27 +5858,25 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
 		 * - bloated or contains data before "start" or
 		 *   overlaps to the next one and mptcp allow collapsing.
 		 */
-		if (!(TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)) &&
-		    (tcp_win_from_space(sk, skb->truesize) > skb->len ||
-		     before(TCP_SKB_CB(skb)->seq, start))) {
-			end_of_skbs = false;
+		if (!skb_frags_readable(skb) ||
+		    TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN))
+			goto skip_this;
+
+		if (tcp_win_from_space(sk, skb->truesize) > skb->len ||
+		    before(TCP_SKB_CB(skb)->seq, start))
 			break;
-		}
 
 		if (n && n != tail && skb_frags_readable(n) &&
 		    tcp_skb_can_collapse_rx(skb, n) &&
-		    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(n)->seq) {
-			end_of_skbs = false;
+		    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(n)->seq)
 			break;
-		}
 
 skip_this:
 		/* Decided to skip this, advance start seq. */
 		start = TCP_SKB_CB(skb)->end_seq;
 	}
-	if (end_of_skbs ||
-	    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)) ||
-	    !skb_frags_readable(skb))
+
+	if (!skb || skb == tail)
 		return;
 
 	__skb_queue_head_init(&tmp);
-- 
2.56.0.611.g1030f0b940-goog