Date: Mon, 3 Jun 2024 18:53:54 +0800
From: shaozhengchao <shaozhengchao@...wei.com>
To: Florian Westphal <fw@...len.de>, <netdev@...r.kernel.org>
CC: Paolo Abeni <pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
	<mleitner@...hat.com>, <juri.lelli@...hat.com>, <vschneid@...hat.com>,
	<tglozar@...hat.com>, <dsahern@...nel.org>, <bigeasy@...utronix.de>,
	<tglx@...utronix.de>
Subject: Re: [PATCH net-next v6 1/3] net: tcp/dcpp: prepare for tw_timer
 un-pinning


The patch header should be "net: tcp/dccp ....".

On 2024/6/3 17:36, Florian Westphal wrote:
> From: Valentin Schneider <vschneid@...hat.com>
> 
> 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
> vs. reschedule and timer firing: timer_del_sync() is replaced with
> timer_shutdown().
> 
> This means that tw_timer may still be running on another CPU.  However, as
> the timer owns a reference on tw sk that is only put at the end this
> should be fine.
> 
> 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@...hat.com>
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>   include/net/inet_timewait_sock.h |  6 ++-
>   net/dccp/minisocks.c             |  3 +-
>   net/ipv4/inet_timewait_sock.c    | 74 ++++++++++++++++++++++++++------
>   net/ipv4/tcp_minisocks.c         |  3 +-
>   4 files changed, 68 insertions(+), 18 deletions(-)
> 
> diff --git a/include/net/inet_timewait_sock.h b/include/net/inet_timewait_sock.h
> index 2a536eea9424..5b43d220243d 100644
> --- a/include/net/inet_timewait_sock.h
> +++ b/include/net/inet_timewait_sock.h
> @@ -93,8 +93,10 @@ struct inet_timewait_sock *inet_twsk_alloc(const struct sock *sk,
>   					   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);
> diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
> index 251a57cf5822..deb52d7d31b4 100644
> --- a/net/dccp/minisocks.c
> +++ b/net/dccp/minisocks.c
> @@ -59,11 +59,10 @@ void dccp_time_wait(struct sock *sk, int state, int timeo)
>   		 * 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
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index e28075f0006e..2b1d836df64e 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -44,14 +44,14 @@ void inet_twsk_bind_unhash(struct inet_timewait_sock *tw,
>   	__sock_put((struct sock *)tw);
>   }
>   
> -/* Must be called with locally disabled BHs. */
> -static void inet_twsk_kill(struct inet_timewait_sock *tw)
> +static void __inet_twsk_kill(struct inet_timewait_sock *tw, spinlock_t *lock)
> +__releases(lock)
>   {
>   	struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
> -	spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
>   	struct inet_bind_hashbucket *bhead, *bhead2;
>   
> -	spin_lock(lock);
> +	lockdep_assert_held(lock);
> +
>   	sk_nulls_del_node_init_rcu((struct sock *)tw);
>   	spin_unlock(lock);
>   
> @@ -71,6 +71,16 @@ static void inet_twsk_kill(struct inet_timewait_sock *tw)
>   	inet_twsk_put(tw);
>   }
>   
> +/* Must be called with locally disabled BHs. */
> +static void inet_twsk_kill(struct inet_timewait_sock *tw)
> +{
> +	struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
> +	spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
> +
> +	spin_lock(lock);
> +	__inet_twsk_kill(tw, lock);
> +}
> +
>   void inet_twsk_free(struct inet_timewait_sock *tw)
>   {
>   	struct module *owner = tw->tw_prot->owner;
> @@ -96,9 +106,13 @@ static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
>    * 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 +143,33 @@ 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 */
>   	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)
>   {
> @@ -217,8 +238,37 @@ EXPORT_SYMBOL_GPL(inet_twsk_alloc);
>    */
>   void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
>   {
> -	if (del_timer_sync(&tw->tw_timer))
> -		inet_twsk_kill(tw);
> +	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).
> +	 *
> +	 * With plain timer_shutdown_sync() and without grabbing the ehash
> +	 * lock, we can 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);
> +	if (timer_shutdown(&tw->tw_timer)) {
> +		/* releases @lock */
> +		__inet_twsk_kill(tw, lock);
> +	} else {
> +		spin_unlock(lock);
> +	}
> +
>   	inet_twsk_put(tw);
>   }
>   EXPORT_SYMBOL(inet_twsk_deschedule_put);
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index 538c06f95918..47de6f3efc85 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -344,11 +344,10 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
>   		 * 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

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