Subject: Re: [PATCH] net: fix race in the receive/select

From: Oleg Nesterov <[email-masked]>
Date: Thu, 25 Jun 2009 14:24:29 +0200

Can't really comment this patch, except this all looks reasonable to me.
Add more CCs.

On 06/25, Jiri Olsa wrote:
>
> Adding memory barrier to the __pollwait function paired with
> receive callbacks.  The smp_mb__after_lock define is added,
> since {read|write|spin}_lock() on x86 are full memory barriers.
>
> The race fires, when following code paths meet, and the tp->rcv_nxt and
> __add_wait_queue updates stay in CPU caches.
>
>
> CPU1                         CPU2
>
> sys_select                   receive packet
>   ...                        ...
>   __add_wait_queue           update tp->rcv_nxt
>   ...                        ...
>   tp->rcv_nxt check          sock_def_readable
>   ...                        {
>   schedule                      ...
>                                 if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
>                                         wake_up_interruptible(sk->sk_sleep)
>                                 ...
>                              }
>
> If there was no cache the code would work ok, since the wait_queue and
> rcv_nxt are opposit to each other.
>
> Meaning that once tp->rcv_nxt is updated by CPU2, the CPU1 either already
> passed the tp->rcv_nxt check and sleeps, or will get the new value for
> tp->rcv_nxt and will return with new data mask.
> In both cases the process (CPU1) is being added to the wait queue, so the
> waitqueue_active (CPU2) call cannot miss and will wake up CPU1.
>
> The bad case is when the __add_wait_queue changes done by CPU1 stay in its
> cache, and so does the tp->rcv_nxt update on CPU2 side.  The CPU1 will then
> endup calling schedule and sleep forever if there are no more data on the
> socket.
>
> wbr,
> jirka
>
>
> Signed-off-by: Jiri Olsa <[email-masked]>
>
> ---
>  arch/x86/include/asm/spinlock.h |    3 +++
>  fs/select.c                     |    4 ++++
>  include/linux/spinlock.h        |    5 +++++
>  include/net/sock.h              |   18 ++++++++++++++++++
>  net/atm/common.c                |    4 ++--
>  net/core/sock.c                 |    8 ++++----
>  net/dccp/output.c               |    2 +-
>  net/iucv/af_iucv.c              |    2 +-
>  net/rxrpc/af_rxrpc.c            |    2 +-
>  net/unix/af_unix.c              |    2 +-
>  10 files changed, 40 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index b7e5db8..39ecc5f 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
>  #define _raw_read_relax(lock)	cpu_relax()
>  #define _raw_write_relax(lock)	cpu_relax()
>
> +/* The {read|write|spin}_lock() on x86 are full memory barriers. */
> +#define smp_mb__after_lock() do { } while (0)
> +
>  #endif /* _ASM_X86_SPINLOCK_H */
> diff --git a/fs/select.c b/fs/select.c
> index d870237..c4bd5f0 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -219,6 +219,10 @@ static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
>  	init_waitqueue_func_entry(&entry->wait, pollwake);
>  	entry->wait.private = pwq;
>  	add_wait_queue(wait_address, &entry->wait);
> +
> +	/* This memory barrier is paired with the smp_mb__after_lock
> +	 * in the sk_has_sleeper. */
> +	smp_mb();
>  }
>
>  int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
> diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> index 252b245..ae053bd 100644
> --- a/include/linux/spinlock.h
> +++ b/include/linux/spinlock.h
> @@ -132,6 +132,11 @@ do {								\
>  #endif /*__raw_spin_is_contended*/
>  #endif
>
> +/* The lock does not imply full memory barrier. */
> +#ifndef smp_mb__after_lock
> +#define smp_mb__after_lock() smp_mb()
> +#endif
> +
>  /**
>   * spin_unlock_wait - wait until the spinlock gets unlocked
>   * @lock: the spinlock in question.
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 352f06b..7fbb143 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1241,6 +1241,24 @@ static inline int sk_has_allocations(const struct sock *sk)
>  	return sk_wmem_alloc_get(sk) || sk_rmem_alloc_get(sk);
>  }
>
> +/**
> + * sk_has_sleeper - check if there are any waiting processes
> + * @sk: socket
> + *
> + * Returns true if socket has waiting processes
> + */
> +static inline int sk_has_sleeper(struct sock *sk)
> +{
> +	/*
> +	 * We need to be sure we are in sync with the
> +	 * add_wait_queue modifications to the wait queue.
> +	 *
> +	 * This memory barrier is paired in the __pollwait.
> +	 */
> +	smp_mb__after_lock();
> +	return sk->sk_sleep && waitqueue_active(sk->sk_sleep);
> +}
> +
>  /*
>   * 	Queue a received datagram if it will fit. Stream and sequenced
>   *	protocols can't normally use this as they need to fit buffers in
> diff --git a/net/atm/common.c b/net/atm/common.c
> index c1c9793..67a8642 100644
> --- a/net/atm/common.c
> +++ b/net/atm/common.c
> @@ -92,7 +92,7 @@ static void vcc_sock_destruct(struct sock *sk)
>  static void vcc_def_wakeup(struct sock *sk)
>  {
>  	read_lock(&sk->sk_callback_lock);
> -	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +	if (sk_has_sleeper(sk))
>  		wake_up(sk->sk_sleep);
>  	read_unlock(&sk->sk_callback_lock);
>  }
> @@ -110,7 +110,7 @@ static void vcc_write_space(struct sock *sk)
>  	read_lock(&sk->sk_callback_lock);
>
>  	if (vcc_writable(sk)) {
> -		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +		if (sk_has_sleeper(sk))
>  			wake_up_interruptible(sk->sk_sleep);
>
>  		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
> diff --git a/net/core/sock.c b/net/core/sock.c
> index b0ba569..6354863 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1715,7 +1715,7 @@ EXPORT_SYMBOL(sock_no_sendpage);
>  static void sock_def_wakeup(struct sock *sk)
>  {
>  	read_lock(&sk->sk_callback_lock);
> -	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +	if (sk_has_sleeper(sk))
>  		wake_up_interruptible_all(sk->sk_sleep);
>  	read_unlock(&sk->sk_callback_lock);
>  }
> @@ -1723,7 +1723,7 @@ static void sock_def_wakeup(struct sock *sk)
>  static void sock_def_error_report(struct sock *sk)
>  {
>  	read_lock(&sk->sk_callback_lock);
> -	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +	if (sk_has_sleeper(sk))
>  		wake_up_interruptible_poll(sk->sk_sleep, POLLERR);
>  	sk_wake_async(sk, SOCK_WAKE_IO, POLL_ERR);
>  	read_unlock(&sk->sk_callback_lock);
> @@ -1732,7 +1732,7 @@ static void sock_def_error_report(struct sock *sk)
>  static void sock_def_readable(struct sock *sk, int len)
>  {
>  	read_lock(&sk->sk_callback_lock);
> -	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +	if (sk_has_sleeper(sk))
>  		wake_up_interruptible_sync_poll(sk->sk_sleep, POLLIN |
>  						POLLRDNORM | POLLRDBAND);
>  	sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
> @@ -1747,7 +1747,7 @@ static void sock_def_write_space(struct sock *sk)
>  	 * progress.  --DaveM
>  	 */
>  	if ((atomic_read(&sk->sk_wmem_alloc) << 1) <= sk->sk_sndbuf) {
> -		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +		if (sk_has_sleeper(sk))
>  			wake_up_interruptible_sync_poll(sk->sk_sleep, POLLOUT |
>  						POLLWRNORM | POLLWRBAND);
>
> diff --git a/net/dccp/output.c b/net/dccp/output.c
> index c0e88c1..c96119f 100644
> --- a/net/dccp/output.c
> +++ b/net/dccp/output.c
> @@ -196,7 +196,7 @@ void dccp_write_space(struct sock *sk)
>  {
>  	read_lock(&sk->sk_callback_lock);
>
> -	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +	if (sk_has_sleeper(sk))
>  		wake_up_interruptible(sk->sk_sleep);
>  	/* Should agree with poll, otherwise some programs break */
>  	if (sock_writeable(sk))
> diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
> index 6be5f92..ba0149d 100644
> --- a/net/iucv/af_iucv.c
> +++ b/net/iucv/af_iucv.c
> @@ -306,7 +306,7 @@ static inline int iucv_below_msglim(struct sock *sk)
>  static void iucv_sock_wake_msglim(struct sock *sk)
>  {
>  	read_lock(&sk->sk_callback_lock);
> -	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +	if (sk_has_sleeper(sk))
>  		wake_up_interruptible_all(sk->sk_sleep);
>  	sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
>  	read_unlock(&sk->sk_callback_lock);
> diff --git a/net/rxrpc/af_rxrpc.c b/net/rxrpc/af_rxrpc.c
> index eac5e7b..60e0e38 100644
> --- a/net/rxrpc/af_rxrpc.c
> +++ b/net/rxrpc/af_rxrpc.c
> @@ -63,7 +63,7 @@ static void rxrpc_write_space(struct sock *sk)
>  	_enter("%p", sk);
>  	read_lock(&sk->sk_callback_lock);
>  	if (rxrpc_writable(sk)) {
> -		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +		if (sk_has_sleeper(sk))
>  			wake_up_interruptible(sk->sk_sleep);
>  		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
>  	}
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 36d4e44..143143a 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -315,7 +315,7 @@ static void unix_write_space(struct sock *sk)
>  {
>  	read_lock(&sk->sk_callback_lock);
>  	if (unix_writable(sk)) {
> -		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
> +		if (sk_has_sleeper(sk))
>  			wake_up_interruptible_sync(sk->sk_sleep);
>  		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
>  	}