tcp_hpts: let tcp_hpts_init() set a random CPU only once

After d2ef52e the tcp_hpts_init() function can be called multiple
times on a tcpcb if it is switched there and back between two TCP stacks.
First, this makes existing assertion in tcp_hpts_init() incorrect.  Second,
it creates possibility to change a randomly set t_hpts_cpu to a different
random value, while a tcpcb is already in the HPTS wheel, triggering other
assertions later in tcp_hptsi().

The best approach here would be to work on the stacks to really clear a
tcpcb out of HPTS wheel in tfb_tcp_fb_fini, draining the IHPTS_MOVING
state.  But that's pretty intrusive change, so let's just get back to the
old logic (pre d2ef52e) where t_hpts_cpu was set to a random value
only once in a CPU lifetime and a newly switched stack inherits t_hpts_cpu
from the previous stack.

Reviewed by:		rrs, tuexen
Differential Revision:	https://reviews.freebsd.org/D42946
Reported-by:	syzbot+fab29fe1ab089c52998d@syzkaller.appspotmail.com
Reported-by:	syzbot+ca5f2aa0fda15dcfe6d7@syzkaller.appspotmail.com
Fixes:		2b3a774

sys/netinet/tcp_hpts.c

@@ -542,15 +542,23 @@ tcp_hpts_release(struct tcpcb *tp)
 }
 
 /*
- * Initialize newborn tcpcb to get ready for use with HPTS.
+ * Initialize tcpcb to get ready for use with HPTS.  We will know which CPU
+ * is preferred on the first incoming packet.  Before that avoid crowding
+ * a single CPU with newborn connections and use a random one.
+ * This initialization is normally called on a newborn tcpcb, but potentially
+ * can be called once again if stack is switched.  In that case we inherit CPU
+ * that the previous stack has set, be it random or not.  In extreme cases,
+ * e.g. syzkaller fuzzing, a tcpcb can already be in HPTS in IHPTS_MOVING state
+ * and has never received a first packet.
  */
 void
 tcp_hpts_init(struct tcpcb *tp)
 {
 
-	tp->t_hpts_cpu = hpts_random_cpu();
-	tp->t_lro_cpu = HPTS_CPU_NONE;
-	MPASS(!(tp->t_flags2 & TF2_HPTS_CPU_SET));
+	if (__predict_true(tp->t_hpts_cpu == HPTS_CPU_NONE)) {
+		tp->t_hpts_cpu = hpts_random_cpu();
+		MPASS(!(tp->t_flags2 & TF2_HPTS_CPU_SET));
+	}
 }
 
 /*

sys/netinet/tcp_subr.c

@@ -2274,6 +2274,9 @@ tcp_newtcpcb(struct inpcb *inp)
 	/* All mbuf queue/ack compress flags should be off */
 	tcp_lro_features_off(tp);
 
+	tp->t_hpts_cpu = HPTS_CPU_NONE;
+	tp->t_lro_cpu = HPTS_CPU_NONE;
+
 	callout_init_rw(&tp->t_callout, &inp->inp_lock, CALLOUT_RETURNUNLOCKED);
 	for (int i = 0; i < TT_N; i++)
 		tp->t_timers[i] = SBT_MAX;