[RFC PATCH] tcp/dcpp: Un-pin tw_timer

[Valentin Schneider]

The TCP timewait timer is proving to be problematic for setups where scheduler
CPU isolation is achieved at runtime via cpusets (as opposed to statically via
isolcpus=domains).

What happens there is a CPU goes through tcp_time_wait(), arming the time_wait
timer, then gets isolated. TCP_TIMEWAIT_LEN later, the timer fires, causing
interference for the now-isolated CPU. This is conceptually similar to the issue
described in
  e02b93124855 ("workqueue: Unbind kworkers before sending them to exit()")

Making the timer un-pinned would resolve this, as it would be queued onto
HK_FLAG_TIMER CPUs. It would Unfortunately go against
  ed2e92394589 ("tcp/dccp: fix timewait races in timer handling")
as we'd need to arm the timer after the *hashdance() to not have it fire before
we've finished setting up the timewait_socket.

However, looking into this, I cannot grok what race is fixed by having the timer
*armed* before the hashdance.

[this next segment is brought to you by Cunningham's Law]

Using [1] as an example, inet_twsk_schedule() only arms the timer and increments
the deathrow refcount, which by itself does not affect
__inet_lookup_established(). AFAICT it only comes in handy if:
1) A CPU ends up livelocking in __inet_lookup_established() (cf. [1], though per
   inet_twsk_alloc() I don't see how a timewait socket can hit the
   forever-looping conditions with how the sk_hash and addr/port pairs are copied)
2) the initialization context can be interrupted by NET_RX (it can, cf.
   cfac7f836a71 ("tcp/dccp: block bh before arming time_wait timer"))

In this scenario, we need the timer to fire to go through
  inet_twsk_kill()
    sk_nulls_del_node_init_rcu()
and break out of the loop.

Keep softirqs disabled, but make the timer un-pinned and arm it after the
hashdance. Remote CPUs may start using the timewait socket before the timer is
armed, but their execution of __inet_lookup_established() won't prevent the
arming of the timer.

This partially reverts
  ed2e92394589 ("tcp/dccp: fix timewait races in timer handling")
and
  ec94c2696f0b ("tcp/dccp: avoid one atomic operation for timewait hashdance")

Link: [1] https://lore.kernel.org/lkml/56941035.9040000@xxxxxxxxxx/
Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/
Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
 net/dccp/minisocks.c          | 18 ++++++++++--------
 net/ipv4/inet_timewait_sock.c |  9 ++++-----
 net/ipv4/tcp_minisocks.c      | 18 ++++++++++--------
 3 files changed, 24 insertions(+), 21 deletions(-)

diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
index 64d805b27adde..188a29a1aef49 100644
--- a/net/dccp/minisocks.c
+++ b/net/dccp/minisocks.c
@@ -53,16 +53,18 @@ void dccp_time_wait(struct sock *sk, int state, int timeo)
 		if (state == DCCP_TIME_WAIT)
 			timeo = DCCP_TIMEWAIT_LEN;
 
-		/* tw_timer is pinned, so we need to make sure BH are disabled
-		 * in following section, otherwise timer handler could run before
-		 * we complete the initialization.
-		 */
-		local_bh_disable();
-		inet_twsk_schedule(tw, timeo);
-		/* Linkage updates.
-		 * Note that access to tw after this point is illegal.
+		/* tw_timer is armed after the hashdance and recount update, so
+		 * we need to make sure BH are disabled in following section to
+		 * ensure the timer is armed before we handle any further skb's.
 		 */
+	       local_bh_disable();
+
+		// Linkage updates
 		inet_twsk_hashdance(tw, sk, &dccp_hashinfo);
+		inet_twsk_schedule(tw, timeo);
+		// Access to tw after this point is illegal.
+		inet_twsk_put(tw);
+
 		local_bh_enable();
 	} else {
 		/* Sorry, if we're out of memory, just CLOSE this
diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
index dd37a5bf68811..ba59c2c6ef4a2 100644
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -152,16 +152,15 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
 
 	spin_unlock(lock);
 
-	/* tw_refcnt is set to 3 because we have :
+	/* tw_refcnt is set to 4 because we have :
 	 * - one reference for bhash chain.
 	 * - one reference for ehash chain.
 	 * - one reference for timer.
+	 * - One reference for ourself (our caller will release it).
 	 * We can use atomic_set() because prior spin_lock()/spin_unlock()
 	 * committed into memory all tw fields.
-	 * Also note that after this point, we lost our implicit reference
-	 * so we are not allowed to use tw anymore.
 	 */
-	refcount_set(&tw->tw_refcnt, 3);
+	refcount_set(&tw->tw_refcnt, 4);
 }
 EXPORT_SYMBOL_GPL(inet_twsk_hashdance);
 
@@ -207,7 +206,7 @@ struct inet_timewait_sock *inet_twsk_alloc(const struct sock *sk,
 		tw->tw_prot	    = sk->sk_prot_creator;
 		atomic64_set(&tw->tw_cookie, atomic64_read(&sk->sk_cookie));
 		twsk_net_set(tw, sock_net(sk));
-		timer_setup(&tw->tw_timer, tw_timer_handler, TIMER_PINNED);
+		timer_setup(&tw->tw_timer, tw_timer_handler, 0);
 		/*
 		 * Because we use RCU lookups, we should not set tw_refcnt
 		 * to a non null value before everything is setup for this
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index b98d476f1594b..269d4aa14a49e 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -324,16 +324,18 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
 		if (state == TCP_TIME_WAIT)
 			timeo = TCP_TIMEWAIT_LEN;
 
-		/* tw_timer is pinned, so we need to make sure BH are disabled
-		 * in following section, otherwise timer handler could run before
-		 * we complete the initialization.
-		 */
-		local_bh_disable();
-		inet_twsk_schedule(tw, timeo);
-		/* Linkage updates.
-		 * Note that access to tw after this point is illegal.
+		/* tw_timer is armed after the hashdance and recount update, so
+		 * we need to make sure BH are disabled in following section to
+		 * ensure the timer is armed before we handle any further skb's.
 		 */
+	       local_bh_disable();
+
+		// Linkage updates.
 		inet_twsk_hashdance(tw, sk, net->ipv4.tcp_death_row.hashinfo);
+		inet_twsk_schedule(tw, timeo);
+		// Access to tw after this point is illegal.
+		inet_twsk_put(tw);
+
 		local_bh_enable();
 	} else {
 		/* Sorry, if we're out of memory, just CLOSE this
-- 
2.39.3