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

From: Eric Dumazet <[email-masked]>
Date: Fri, 26 Jun 2009 03:59:01 +0200

Davide Libenzi a écrit :
> On Thu, 25 Jun 2009, Oleg Nesterov wrote:
> 
>> Can't really comment this patch, except this all looks reasonable to me.
>> Add more CCs.
> 
> While this can work, IMO it'd be cleaner to have the smp_mb() moved from 
> fs/select.c to the ->poll() function.
> Having a barrier that matches another one in another susbsystem, because 
> of the special locking logic of such subsystem, is not too shiny IMHO.
> 

Yes but barrier is necessary only if add_wait_queue() was actually called, and __pollwait()
does this call.

Adding a plain smp_mb() in tcp_poll() for example would slowdown select()/poll() with NULL
timeout.

Adding a cond test before smp_mb() in tcp_poll() (and other ->poll() functions)
would be litle bit overkill too...

I believe this race was not existent in the past because spin_unlock() had a memory barrier,
and we changed this to a plain memory write at some point...

Most add_wait_queue() calls are followed by a call to set_current_state()
so a proper smp_mb() is explicitly included.

> 
> 
> 
>> 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);
>>>  	}
>> --
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>>
> 
> 
> - Davide
> 
>