Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753315AbZFXKUx (ORCPT ); Wed, 24 Jun 2009 06:20:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752074AbZFXKUn (ORCPT ); Wed, 24 Jun 2009 06:20:43 -0400 Received: from mx2.redhat.com ([66.187.237.31]:55593 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750720AbZFXKUl (ORCPT ); Wed, 24 Jun 2009 06:20:41 -0400 Date: Wed, 24 Jun 2009 12:20:38 +0200 From: Jiri Olsa To: Eric Dumazet Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, fbl@redhat.com, nhorman@redhat.com, davem@redhat.com, oleg@redhat.com Subject: Re: [RFC] tcp: race in receive part Message-ID: <20090624102038.GA5409@jolsa.lab.eng.brq.redhat.com> References: <20090618102727.GC3782@jolsa.lab.eng.brq.redhat.com> <4A3A49F2.6060705@gmail.com> <20090623091257.GA4850@jolsa.lab.eng.brq.redhat.com> <4A40AF2A.3050509@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4A40AF2A.3050509@gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 11779 Lines: 328 On Tue, Jun 23, 2009 at 12:32:10PM +0200, Eric Dumazet wrote: > Jiri Olsa a écrit : > > Hi, > > > > thanks for an answer, and sorry for my late reply, > > we needed the cust permission to disclose the debug data. > > > > I see ! Now this is me with litle time as I am traveling right now. > > > > > On Thu, Jun 18, 2009 at 04:06:42PM +0200, Eric Dumazet wrote: > >> 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. " > > > > please try it now, the bug should be accessible now > > > > Thanks, this doc is indeed nice :) > > But adding an write_lock()/write_unlock() in tcp_poll() was overkill > It had an sm_mb() implied because of the nesting of locks. > > >>> > >>> 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 ? > > > > there's a reproducer attached to the bug > > > > https://enterprise.redhat.com/issue-tracker/?module=download&fid=201560&key=f6f87caf6ac2dc1eb1173257c8a5ef78 > > > > it is basically the client/server program. > > They're passing messages to each other. When a message is sent, > > both of them expect message on the input before sending another message. > > > > Very rarely the code hits the place when the process which called select > > is not woken up by incomming data. Probably because of the memory cache > > incoherency. See backtrace in the > > > > https://bugzilla.redhat.com/show_bug.cgi?id=494404#c1 > > > > > >>> 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 > >>> > >>> 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) > >> > > > > First version of the patch was actually in this layer, see > > https://bugzilla.redhat.com/attachment.cgi?id=345886 > > > > I was adviced this could be to invasive (it was in waitqueue_active actually), > > so I moved the change to the TCP layer itself... > > > > As far as I understand the problem there's need for 2 barriers to be > > sure, the memory will have correct data. One in the codepath calling the > > select (tcp_poll), and in the other one updating the available data status > > (sock_def_readable), am I missing smth? > > > > Hmm, I am not saying your patch doesnt fix the problem, I am saying it > is a partial fix of a general problem. We might have same problem(s) in other > parts of network stack. This is a very serious issue. > > Point 1 : > > You added an smp_mb() call in tcp_poll(). This one looks fine to solve > the problem for tcp sockets. What about other protocols ? Do we have > same problem ? Looks like most of the protocols using the poll_wait. Also I assume that most of them will probably have the same scenario as the one described (CPU1 and CPU2 codepaths in the RACE DESCRIPTION). So I moved the poll smp_mb() call to the __pollwait function, plz check the attached diff. This might be too invasive, so another place could be probably polling callbacks themselfs like datagram_poll (used very often by protocols), tcp_poll, udp_poll... I'm still looking which way would be more suitable, comments are very welcome :) > > Point 2 : > > You added several smp_mb() calls in tcp input path. In my first > reply, I said it was probably better to add smp_mb() in a single > place, right before "if (waitqueue_active(sk->sk_sleep) ... ", > but in all paths (input path & output path). > > Point 3 : > > The optimization we could do, defining > a smp_mb_after_read_lock() that could be a nop on x86 > > read_lock(&sk->sk_callback_lock); // "lock subl $0x1,(%eax)" on x86 > smp_mb_after_read_lock(); /* compiler barrier() on x86 */ > if (sk->sk_sleep && waitqueue_active(sk->sk_sleep)) > wake_up_interruptible(sk->sk_sleep); > > Am I missing something ? > > ;) > not at all :) I'm the one behind.. Anyway I made modifications based on Point 2) and 3) and the diff is attached, please check. thanks a lot, jirka diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h index b7e5db8..570c0ff 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_lock() on x86 is a full memory barrier. */ +#define smp_mb__after_read_lock() barrier() + #endif /* _ASM_X86_SPINLOCK_H */ diff --git a/fs/select.c b/fs/select.c index d870237..f9ebd45 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_read_lock + * in the sock_def_readable. */ + 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..dd28726 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -132,6 +132,11 @@ do { \ #endif /*__raw_spin_is_contended*/ #endif +/* The read_lock does not imply full memory barrier. */ +#ifndef smp_mb__after_read_lock +#define smp_mb__after_read_lock() smp_mb() +#endif + /** * spin_unlock_wait - wait until the spinlock gets unlocked * @lock: the spinlock in question. diff --git a/net/core/sock.c b/net/core/sock.c index b0ba569..11e414f 100644 --- a/net/core/sock.c +++ b/net/core/sock.c @@ -1732,6 +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); + smp_mb__after_read_lock(); if (sk->sk_sleep && waitqueue_active(sk->sk_sleep)) wake_up_interruptible_sync_poll(sk->sk_sleep, POLLIN | POLLRDNORM | POLLRDBAND); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/