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From: Valentin Schneider <vschneid@redhat.com>
Date: Tue, 4 Jun 2024 16:08:47 +0200
Subject: [PATCH 1/3] net: tcp/dccp: prepare for tw_timer un-pinning
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.10/older/patches-6.10.2-rt14.tar.xz
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 commit e02b93124855 ("workqueue: Unbind
kworkers before sending them to exit()")
Move inet_twsk_schedule() to within inet_twsk_hashdance(), with the ehash
lock held. Expand the lock's critical section from inet_twsk_kill() to
inet_twsk_deschedule_put(), serializing the scheduling vs descheduling of
the timer. IOW, this prevents the following race:
tcp_time_wait()
inet_twsk_hashdance()
inet_twsk_deschedule_put()
del_timer_sync()
inet_twsk_schedule()
Thanks to Paolo Abeni for suggesting to leverage the ehash lock.
This also restores a comment from commit 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.
inet_twsk_deschedule_put() now acquires the ehash spinlock to synchronize
with inet_twsk_hashdance_schedule().
To ease possible regression search, actual un-pin is done in next patch.
Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20240604140903.31939-2-fw@strlen.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
include/net/inet_timewait_sock.h | 6 +++-
net/dccp/minisocks.c | 3 --
net/ipv4/inet_timewait_sock.c | 52 +++++++++++++++++++++++++++++++++------
net/ipv4/tcp_minisocks.c | 3 --
4 files changed, 51 insertions(+), 13 deletions(-)
--- a/include/net/inet_timewait_sock.h
+++ b/include/net/inet_timewait_sock.h
@@ -93,8 +93,10 @@ struct inet_timewait_sock *inet_twsk_all
struct inet_timewait_death_row *dr,
const int state);
-void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
- struct inet_hashinfo *hashinfo);
+void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
+ struct sock *sk,
+ struct inet_hashinfo *hashinfo,
+ int timeo);
void __inet_twsk_schedule(struct inet_timewait_sock *tw, int timeo,
bool rearm);
--- a/net/dccp/minisocks.c
+++ b/net/dccp/minisocks.c
@@ -59,11 +59,10 @@ void dccp_time_wait(struct sock *sk, int
* we complete the initialization.
*/
local_bh_disable();
- inet_twsk_schedule(tw, timeo);
/* Linkage updates.
* Note that access to tw after this point is illegal.
*/
- inet_twsk_hashdance(tw, sk, &dccp_hashinfo);
+ inet_twsk_hashdance_schedule(tw, sk, &dccp_hashinfo, timeo);
local_bh_enable();
} else {
/* Sorry, if we're out of memory, just CLOSE this
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -96,9 +96,13 @@ static void inet_twsk_add_node_rcu(struc
* Enter the time wait state. This is called with locally disabled BH.
* Essentially we whip up a timewait bucket, copy the relevant info into it
* from the SK, and mess with hash chains and list linkage.
+ *
+ * The caller must not access @tw anymore after this function returns.
*/
-void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
- struct inet_hashinfo *hashinfo)
+void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
+ struct sock *sk,
+ struct inet_hashinfo *hashinfo,
+ int timeo)
{
const struct inet_sock *inet = inet_sk(sk);
const struct inet_connection_sock *icsk = inet_csk(sk);
@@ -129,26 +133,33 @@ void inet_twsk_hashdance(struct inet_tim
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 */
if (__sk_nulls_del_node_init_rcu(sk))
sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
- spin_unlock(lock);
+ /* Ensure above writes are committed into memory before updating the
+ * refcount.
+ * Provides ordering vs later refcount_inc().
+ */
+ smp_wmb();
/* tw_refcnt is set to 3 because we have :
* - one reference for bhash chain.
* - one reference for ehash chain.
* - one reference for timer.
- * 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);
+
+ inet_twsk_schedule(tw, timeo);
+
+ spin_unlock(lock);
}
-EXPORT_SYMBOL_GPL(inet_twsk_hashdance);
+EXPORT_SYMBOL_GPL(inet_twsk_hashdance_schedule);
static void tw_timer_handler(struct timer_list *t)
{
@@ -217,7 +228,34 @@ EXPORT_SYMBOL_GPL(inet_twsk_alloc);
*/
void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
{
- if (del_timer_sync(&tw->tw_timer))
+ struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
+ spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
+
+ /* inet_twsk_purge() walks over all sockets, including tw ones,
+ * and removes them via inet_twsk_deschedule_put() after a
+ * refcount_inc_not_zero().
+ *
+ * inet_twsk_hashdance_schedule() must (re)init the refcount before
+ * arming the timer, i.e. inet_twsk_purge can obtain a reference to
+ * a twsk that did not yet schedule the timer.
+ *
+ * The ehash lock synchronizes these two:
+ * After acquiring the lock, the timer is always scheduled (else
+ * timer_shutdown returns false), because hashdance_schedule releases
+ * the ehash lock only after completing the timer initialization.
+ *
+ * Without grabbing the ehash lock, we get:
+ * 1) cpu x sets twsk refcount to 3
+ * 2) cpu y bumps refcount to 4
+ * 3) cpu y calls inet_twsk_deschedule_put() and shuts timer down
+ * 4) cpu x tries to start timer, but mod_timer is a noop post-shutdown
+ * -> timer refcount is never decremented.
+ */
+ spin_lock(lock);
+ /* Makes sure hashdance_schedule() has completed */
+ spin_unlock(lock);
+
+ if (timer_shutdown_sync(&tw->tw_timer))
inet_twsk_kill(tw);
inet_twsk_put(tw);
}
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -344,11 +344,10 @@ void tcp_time_wait(struct sock *sk, int
* we complete the initialization.
*/
local_bh_disable();
- inet_twsk_schedule(tw, timeo);
/* Linkage updates.
* Note that access to tw after this point is illegal.
*/
- inet_twsk_hashdance(tw, sk, net->ipv4.tcp_death_row.hashinfo);
+ inet_twsk_hashdance_schedule(tw, sk, net->ipv4.tcp_death_row.hashinfo, timeo);
local_bh_enable();
} else {
/* Sorry, if we're out of memory, just CLOSE this
|
