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()")
Keep softirqs disabled, but make the timer un-pinned and arm it *after* the
hashdance.
This introduces the following (non-fatal) race:
CPU0 CPU1
allocates a tw
insert it in hash table
finds the TW and removes it
(timer cancel does nothing)
arms a TW timer, lasting
This partially reverts
ed2e92394589 ("tcp/dccp: fix timewait races in timer handling")
and
ec94c2696f0b ("tcp/dccp: avoid one atomic operation for timewait hashdance")
This also reinstores a comment from
ec94c2696f0b ("tcp/dccp: avoid one atomic operation for timewait hashdance")
as inet_twsk_hashdance() had a "Step 1" and "Step 3" comment, but the "Step
2" had gone missing.
Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/
Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
net/dccp/minisocks.c | 16 +++++++---------
net/ipv4/inet_timewait_sock.c | 20 +++++++++++++++-----
net/ipv4/tcp_minisocks.c | 16 +++++++---------
3 files changed, 29 insertions(+), 23 deletions(-)
diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
index 64d805b27adde..2f0fad4255e36 100644
--- a/net/dccp/minisocks.c
+++ b/net/dccp/minisocks.c
@@ -53,16 +53,14 @@ 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.
- */
+ 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..f9b2bbedf1cfc 100644
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -144,6 +144,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
spin_lock(lock);
+ /* Step 2: Hash TW into tcp ehash chain */
inet_twsk_add_node_rcu(tw, &ehead->chain);
/* Step 3: Remove SK from hash chain */
@@ -152,16 +153,15 @@ 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 +207,7 @@ 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
@@ -232,6 +232,16 @@ EXPORT_SYMBOL_GPL(inet_twsk_alloc);
*/
void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
{
+ /* This can race with tcp_time_wait() and dccp_time_wait(), as the timer
+ * is armed /after/ adding it to the hashtables.
+ *
+ * If this is interleaved between inet_twsk_hashdance() and inet_twsk_put(),
+ * then this is a no-op: the timer will still end up armed.
+ *
+ * Conversely, if this successfully deletes the timer, then we know we
+ * have already gone through {tcp,dcpp}_time_wait(), and we can safely
+ * call inet_twsk_kill().
+ */
if (del_timer_sync(&tw->tw_timer))
inet_twsk_kill(tw);
inet_twsk_put(tw);
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index a9807eeb311ca..48eb0310fe837 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -338,16 +338,14 @@ 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.
- */
+ 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
--
2.41.0
