Messages in this thread | | | Date | Thu, 18 Jun 2009 16:06:42 +0200 | From | Eric Dumazet <> | Subject | Re: [RFC] tcp: race in receive part |
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Jiri Olsa a écrit : > Hi, > > in RHEL4 we can see a race in the tcp layer. We were not able to reproduce > this on the upstream kernel, but since the issue occurs very rarelly > (once per 8 days), we just might not be lucky. > > I'm affraid this might be a long email, I'll try to structure it nicely.. :) >
Thanks for your mail and detailed analysis
> > > RACE DESCRIPTION > ================ > > There's a nice pdf describing the issue (and sollution using locks) on > https://bugzilla.redhat.com/attachment.cgi?id=345014
I could not reach this url unfortunatly
--> "You are not authorized to access bug #494404. "
> > > 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 were 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. > > Adding smp_mb() calls before sock_def_readable call and after __add_wait_queue > should prevent the above bad scenario. > > The upstream patch is attached. It seems to prevent the issue. > > > > CPU BUGS > ======== > > The customer has been able to reproduce this problem only on one CPU model: > Xeon E5345*2. They didn't reproduce on XEON MV, for example.
Is there an easy way to reproduce the problem ?
> > That CPU model happens to have 2 possible issues, that might cause the issue: > (see errata http://www.intel.com/Assets/PDF/specupdate/315338.pdf) > > AJ39 and AJ18. The first one can be workarounded by BIOS upgrade, > the other one has following notes:
AJ18 only matters on unaligned accesses, tcp code doesnt do this.
> > Software should ensure at least one of the following is true when > modifying shared data by multiple agents: > • The shared data is aligned > • Proper semaphores or barriers are used in order to > prevent concurrent data accesses. > > > > RFC > === > > I'm aware that not having this issue reproduced on upstream lowers the odds > having this checked in. However AFAICS the issue is present. I'd appreciate > any comment/ideas. > > > thanks, > jirka > > > Signed-off-by: Jiri Olsa <jolsa@redhat.com> > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c > index 17b89c5..f5d9dbf 100644 > --- a/net/ipv4/tcp.c > +++ b/net/ipv4/tcp.c > @@ -340,6 +340,11 @@ unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait) > struct tcp_sock *tp = tcp_sk(sk); > > poll_wait(file, sk->sk_sleep, wait);
poll_wait() calls add_wait_queue() which contains a spin_lock_irqsave()/spin_unlock_irqrestore() pair
Documentation/memory-barriers.txt states in line 1123 :
Memory operations issued after the LOCK will be completed after the LOCK operation has completed.
and line 1131 states :
Memory operations issued before the UNLOCK will be completed before the UNLOCK operation has completed.
So yes, there is no full smp_mb() in poll_wait()
> + > + /* Get in sync with tcp_data_queue, tcp_urg > + and tcp_rcv_established function. */ > + smp_mb();
If this barrier is really necessary, I guess it should be done in poll_wait() itself.
Documentation/memory-barriers.txt misses some information about poll_wait()
> + > if (sk->sk_state == TCP_LISTEN) > return inet_csk_listen_poll(sk); > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c > index 2bdb0da..0606e5e 100644 > --- a/net/ipv4/tcp_input.c > +++ b/net/ipv4/tcp_input.c > @@ -4362,8 +4362,11 @@ queue_and_out: > > if (eaten > 0) > __kfree_skb(skb); > - else if (!sock_flag(sk, SOCK_DEAD)) > + else if (!sock_flag(sk, SOCK_DEAD)) { > + /* Get in sync with tcp_poll function. */ > + smp_mb(); > sk->sk_data_ready(sk, 0); > + } > return; >
Oh well... if smp_mb() is needed, I believe it should be done right before "if (waitqueue_active(sk->sk_sleep) ... "
read_lock(&sk->sk_callback_lock); + smp_mb(); if (sk->sk_sleep && waitqueue_active(sk->sk_sleep)) wake_up_interruptible(sk->sk_sleep)
It would match other parts in kernel (see fs/splice.c, fs/aio.c, ...)
Strange thing is that read_lock() on x86 is a full memory barrier, as it uses "lock subl $0x1,(%eax)"
Maybe we could define a smp_mb_after_read_lock() (a compiler barrier() on x86)
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