tcp: use single locked callout per tcpcb for the TCP timers

Use only one callout structure per tcpcb that is responsible for handling
all five TCP timeouts.  Use locked version of callout, of course. The
callout function tcp_timer_enter() chooses soonest timer and executes it
with lock held.  Unless the timer reports that the tcpcb has been freed,
the callout is rescheduled for next soonest timer, if there is any.

With single callout per tcpcb on connection teardown we should be able
to fully stop the callout and immediately free it, avoiding use of
callout_async_drain().  There is one gotcha here: callout_stop() can
actually touch our memory when a rare race condition happens.  See
comment above tcp_timer_stop().  Synchronous stop of the callout makes
tcp_discardcb() the single entry point for tcpcb destructor, merging the
tcp_freecb() to the end of the function.

While here, also remove lots of lingering checks in the beginning of
TCP timer functions.  With a locked callout they are unnecessary.

While here, clean unused parts of timer KPI for the pluggable TCP stacks.

While here, remove TCPDEBUG from tcp_timer.c, as this allows for more
simplification of TCP timers.  The TCPDEBUG is scheduled for removal.

Move the DTrace probes in timers to the beginning of a function, where
a tcpcb is always existing.

Discussed with:		rrs, tuexen, rscheff	(the TCP part of the diff)
Reviewed by:		hselasky, kib, mav	(the callout part)
Differential revision:	https://reviews.freebsd.org/D37321

1 files changed, 2 insertions, 29 deletions

diff --git a/sys/netinet/tcp_stacks/bbr.c b/sys/netinet/tcp_stacks/bbr.c
index edba270c151b..5a1e3de4c416 100644
--- a/sys/netinet/tcp_stacks/bbr.c
+++ b/sys/netinet/tcp_stacks/bbr.c
@@ -5285,37 +5285,13 @@ bbr_timer_cancel(struct tcp_bbr *bbr, int32_t line, uint32_t cts)
 	}
 }
 
-static void
-bbr_timer_stop(struct tcpcb *tp, uint32_t timer_type)
+static int
+bbr_stopall(struct tcpcb *tp)
 {
 	struct tcp_bbr *bbr;
 
 	bbr = (struct tcp_bbr *)tp->t_fb_ptr;
 	bbr->rc_all_timers_stopped = 1;
-	return;
-}
-
-/*
- * stop all timers always returning 0.
- */
-static int
-bbr_stopall(struct tcpcb *tp)
-{
-	return (0);
-}
-
-static void
-bbr_timer_activate(struct tcpcb *tp, uint32_t timer_type, uint32_t delta)
-{
-	return;
-}
-
-/*
- * return true if a bbr timer (rack or tlp) is active.
- */
-static int
-bbr_timer_active(struct tcpcb *tp, uint32_t timer_type)
-{
 	return (0);
 }
 
@@ -14168,9 +14144,6 @@ struct tcp_function_block __tcp_bbr = {
 	.tfb_tcp_fb_init = bbr_init,
 	.tfb_tcp_fb_fini = bbr_fini,
 	.tfb_tcp_timer_stop_all = bbr_stopall,
-	.tfb_tcp_timer_activate = bbr_timer_activate,
-	.tfb_tcp_timer_active = bbr_timer_active,
-	.tfb_tcp_timer_stop = bbr_timer_stop,
 	.tfb_tcp_rexmit_tmr = bbr_remxt_tmr,
 	.tfb_tcp_handoff_ok = bbr_handoff_ok,
 	.tfb_tcp_mtu_chg = bbr_mtu_chg,