bgpd: Fix deadlock in bgp_keepalive and master pthreads

(gdb) bt
0  futex_wait (private=0, expected=2, futex_word=0x5c438e9a98d8) at ../sysdeps/nptl/futex-internal.h:146
1  __GI___lll_lock_wait (futex=futex@entry=0x5c438e9a98d8, private=0) at ./nptl/lowlevellock.c:49
2  0x00007af16d698002 in lll_mutex_lock_optimized (mutex=0x5c438e9a98d8) at ./nptl/pthread_mutex_lock.c:48
3  ___pthread_mutex_lock (mutex=0x5c438e9a98d8) at ./nptl/pthread_mutex_lock.c:93
4  0x00005c4369c17e70 in _frr_mtx_lock (mutex=0x5c438e9a98d8, func=0x5c4369dc2750 <__func__.265> "bgp_notify_send_internal") at ./lib/frr_pthread.h:258
5  0x00005c4369c1a07a in bgp_notify_send_internal (connection=0x5c438e9a98c0, code=8 '\b', sub_code=0 '\000', data=0x0, datalen=0, use_curr=true) at bgpd/bgp_packet.c:928
6  0x00005c4369c1a707 in bgp_notify_send (connection=0x5c438e9a98c0, code=8 '\b', sub_code=0 '\000') at bgpd/bgp_packet.c:1069
7  0x00005c4369bea422 in bgp_stop_with_notify (connection=0x5c438e9a98c0, code=8 '\b', sub_code=0 '\000') at bgpd/bgp_fsm.c:1597
8  0x00005c4369c18480 in bgp_packet_add (connection=0x5c438e9a98c0, peer=0x5c438e9b6010, s=0x7af15c06bf70) at bgpd/bgp_packet.c:151
9  0x00005c4369c19816 in bgp_keepalive_send (peer=0x5c438e9b6010) at bgpd/bgp_packet.c:639
10 0x00005c4369bf01fd in peer_process (hb=0x5c438ed05520, arg=0x7af16bdffaf0) at bgpd/bgp_keepalives.c:111
11 0x00007af16dacd8e6 in hash_iterate (hash=0x7af15c000be0, func=0x5c4369bf005e <peer_process>, arg=0x7af16bdffaf0) at lib/hash.c:252
12 0x00005c4369bf0679 in bgp_keepalives_start (arg=0x5c438e0db110) at bgpd/bgp_keepalives.c:214
13 0x00007af16dac9932 in frr_pthread_inner (arg=0x5c438e0db110) at lib/frr_pthread.c:180
14 0x00007af16d694ac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
15 0x00007af16d726850 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
(gdb)

The bgp keepalive pthread gets deadlocked with itself and consequently
the bgp master pthread gets locked when it attempts to lock
the peerhash_mtx, since it is also locked by the keepalive_pthread

The keepalive pthread is locking the peerhash_mtx in
bgp_keepalives_start.  Next the connection->io_mtx mutex in
bgp_keepalives_send is locked and then when it notices a problem it invokes
bgp_stop_with_notify which relocks the same mutex ( and of course
the relock causes it to get stuck on itself ).  This generates a
deadlock condition.

Modify the code to only hold the connection->io_mtx as short as
possible.

Signed-off-by: Donald Sharp <[email protected]>

bgpd/bgp_packet.c

@@ -122,41 +122,39 @@ static void bgp_packet_add(struct peer_connection *connection,
 			peer->last_sendq_ok = monotime(NULL);
 
 		stream_fifo_push(connection->obuf, s);
+	}
 
-		delta = monotime(NULL) - peer->last_sendq_ok;
+	delta = monotime(NULL) - peer->last_sendq_ok;
 
-		if (CHECK_FLAG(peer->flags, PEER_FLAG_TIMER))
-			holdtime = atomic_load_explicit(&peer->holdtime,
-							memory_order_relaxed);
-		else
-			holdtime = peer->bgp->default_holdtime;
+	if (CHECK_FLAG(peer->flags, PEER_FLAG_TIMER))
+		holdtime = atomic_load_explicit(&peer->holdtime,
+						memory_order_relaxed);
+	else
+		holdtime = peer->bgp->default_holdtime;
 
-		sendholdtime = holdtime * 2;
+	sendholdtime = holdtime * 2;
 
-		/* Note that when we're here, we're adding some packet to the
-		 * OutQ.  That includes keepalives when there is nothing to
-		 * do, so there's a guarantee we pass by here once in a while.
-		 *
-		 * That implies there is no need to go set up another separate
-		 * timer that ticks down SendHoldTime, as we'll be here sooner
-		 * or later anyway and will see the checks below failing.
-		 */
-		if (!holdtime) {
-			/* no holdtime, do nothing. */
-		} else if (delta > sendholdtime) {
-			flog_err(
-				EC_BGP_SENDQ_STUCK_PROPER,
-				"%pBP has not made any SendQ progress for 2 holdtimes (%jds), terminating session",
-				peer, sendholdtime);
-			BGP_EVENT_ADD(connection, TCP_fatal_error);
-		} else if (delta > (intmax_t)holdtime &&
-			   monotime(NULL) - peer->last_sendq_warn > 5) {
-			flog_warn(
-				EC_BGP_SENDQ_STUCK_WARN,
-				"%pBP has not made any SendQ progress for 1 holdtime (%us), peer overloaded?",
-				peer, holdtime);
-			peer->last_sendq_warn = monotime(NULL);
-		}
+	/* Note that when we're here, we're adding some packet to the
+	 * OutQ.  That includes keepalives when there is nothing to
+	 * do, so there's a guarantee we pass by here once in a while.
+	 *
+	 * That implies there is no need to go set up another separate
+	 * timer that ticks down SendHoldTime, as we'll be here sooner
+	 * or later anyway and will see the checks below failing.
+	 */
+	if (!holdtime) {
+		/* no holdtime, do nothing. */
+	} else if (delta > sendholdtime) {
+		flog_err(EC_BGP_SENDQ_STUCK_PROPER,
+			 "%pBP has not made any SendQ progress for 2 holdtimes (%jds), terminating session",
+			 peer, sendholdtime);
+		BGP_EVENT_ADD(connection, TCP_fatal_error);
+	} else if (delta > (intmax_t)holdtime &&
+		   monotime(NULL) - peer->last_sendq_warn > 5) {
+		flog_warn(EC_BGP_SENDQ_STUCK_WARN,
+			  "%pBP has not made any SendQ progress for 1 holdtime (%us), peer overloaded?",
+			  peer, holdtime);
+		peer->last_sendq_warn = monotime(NULL);
 	}
 }