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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , mleitner@redhat.com, David Ahern , Juri Lelli , Tomas Glozar , Sebastian Andrzej Siewior , Thomas Gleixner Subject: [PATCH v3 1/1] tcp/dcpp: Un-pin tw_timer Date: Mon, 19 Feb 2024 10:57:29 +0100 Message-ID: <20240219095729.2339914-2-vschneid@redhat.com> In-Reply-To: <20240219095729.2339914-1-vschneid@redhat.com> References: <20240219095729.2339914-1-vschneid@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1791320858274395137 X-GMAIL-MSGID: 1791320915202709543 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 --- 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 5befa4de5b241..61a053fbd329c 100644 --- a/net/ipv4/inet_timewait_sock.c +++ b/net/ipv4/inet_timewait_sock.c @@ -129,6 +129,7 @@ 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 */ @@ -137,16 +138,15 @@ 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); @@ -192,7 +192,7 @@ 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 @@ -217,6 +217,16 @@ 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 9e85f2a0bddd4..54e025ba9b015 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