net: count last skb of each sk_backlog pass in __release_sock()

Running 500 concurrent TCPv6 streams (neper "tcp_stream -6 -T 1 -F 500")
on a 2-core Cortex-A53 (CONFIG_PREEMPT_NONE=y, HZ=1000, LATENCY_WARN)
triggers resched_latency_warn() in __release_sock():

  [ 3655.311821] sched: CPU 0 need_resched set for > 100006892 ns (100 ticks) without schedule
  [ 3655.311889] Call trace:
  ...
  [ 3655.311944]  resched_latency_warn+0x7c/0x88
  [ 3655.311958]  sched_tick+0x1c0/0x418
  ...
  [ 3655.312578]  net_rx_action+0x238/0x438
  ...
  [ 3655.312641]  do_softirq+0x4c/0x90
  [ 3655.312651]  __local_bh_enable_ip+0x84/0x90
  [ 3655.312663]  local_bh_enable+0x1c/0x28
  [ 3655.312674]  __dev_queue_xmit+0x4a8/0xb00
  ...
  [ 3655.312782]  tcp_v6_do_rcv+0x240/0x5f8
  [ 3655.312793]  __release_sock+0x9c/0x188
  [ 3655.312806]  release_sock+0x150/0x168
  [ 3655.312817]  tcp_sendmsg+0x4c/0x70

__release_sock() can loop past 100 ms without calling cond_resched():

- Each outer-loop pass drains sk_backlog, but "if (!skb) break;" runs
  before "++nb", so a pass of B skbs only adds B - 1 to nb (0 for 1 skb).
- With 500 TCPv6 streams and NAPI sharing two Cortex-A53 cores, runnable
  tasks keep TIF_NEED_RESCHED set on CPU 0.
- During spin_unlock_bh() and TX softirqs (__dev_queue_xmit() ->
  local_bh_enable()), tcp_add_backlog() coalesces arriving pure ACKs into
  a single tail skb.
- Repeated 1-skb passes never advance nb or call cond_resched(),
  triggering resched_latency_warn() after 100 ticks.

Restore the do-while loop and drop "if (!skb) break;" so every skb
increments nb across passes and calls cond_resched() every 16 skbs.

Reproduced the issue and verified the fix on two 2-core Cortex-A53 hosts
(CONFIG_PREEMPT_NONE=y, HZ=1000, LATENCY_WARN, latency_warn_once=0) with
a userspace reproducer over /dev/net/tun:

- CPU 0 runs a TCPv6 sender and a second runnable task while CPU 1 writes
  pure ACKs to /dev/net/tun (~30k TCPBacklogCoalesce/run), sustaining
  1-skb sk_backlog refills in __release_sock() for ~400 ms (ftrace).
- Before: CPU 0 stayed unscheduled in __release_sock(), hitting
  resched_latency_warn() 293 and 275 times on the two hosts (> 100.9 ms).
- After: cond_resched() yielded CPU 0 inside __release_sock(), with
  0 resched_latency_warn() hits on both hosts.
- Also observed no regression with the workload from
  commit 16c610162d ("net: call cond_resched() less often in
  __release_sock()"), "super_netperf 4 -H srv -T,10 -l 3000 &".

Fixes: 16c610162d ("net: call cond_resched() less often in __release_sock()")
Cc: stable@vger.kernel.org
Signed-off-by: Ray Zhang <sgzhang@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20261101075145.698399-1-sgzhang@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/sock.c

@@ -3333,7 +3333,7 @@ void __release_sock(struct sock *sk)
 
 		spin_unlock_bh(&sk->sk_lock.slock);
 
-		while (1) {
+		do {
 			next = skb->next;
 			prefetch(next);
 			DEBUG_NET_WARN_ON_ONCE(skb_dst_is_noref(skb));
@@ -3341,12 +3341,10 @@ void __release_sock(struct sock *sk)
 			sk_backlog_rcv(sk, skb);
 
 			skb = next;
-			if (!skb)
-				break;
 
 			if (!(++nb & 15))
 				cond_resched();
-		}
+		} while (skb);
 
 		spin_lock_bh(&sk->sk_lock.slock);
 	}