Re: [PATCH v2] sunrpc: fix waitqueue_active without memory barrierin sunrpc

[Kosuke Tatsukawa]

J. Bruce Fields wrote:
> On Wed, Oct 14, 2015 at 03:57:13AM +0000, Kosuke Tatsukawa wrote:
>> J. Bruce Fields wrote:
>> > On Mon, Oct 12, 2015 at 10:41:06AM +0000, Kosuke Tatsukawa wrote:
>> >> J. Bruce Fields wrote:
>> >> > On Fri, Oct 09, 2015 at 06:29:44AM +0000, Kosuke Tatsukawa wrote:
>> >> >> Neil Brown wrote:
>> >> >> > Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx> writes:
>> >> >> > 
>> >> >> >> There are several places in net/sunrpc/svcsock.c which calls
>> >> >> >> waitqueue_active() without calling a memory barrier.  Add a memory
>> >> >> >> barrier just as in wq_has_sleeper().
>> >> >> >>
>> >> >> >> I found this issue when I was looking through the linux source code
>> >> >> >> for places calling waitqueue_active() before wake_up*(), but without
>> >> >> >> preceding memory barriers, after sending a patch to fix a similar
>> >> >> >> issue in drivers/tty/n_tty.c  (Details about the original issue can be
>> >> >> >> found here: https://lkml.org/lkml/2015/9/28/849).
>> >> >> > 
>> >> >> > hi,
>> >> >> > this feels like the wrong approach to the problem.  It requires extra
>> >> >> > 'smb_mb's to be spread around which are hard to understand as easy to
>> >> >> > forget.
>> >> >> > 
>> >> >> > A quick look seems to suggest that (nearly) every waitqueue_active()
>> >> >> > will need an smb_mb.  Could we just put the smb_mb() inside
>> >> >> > waitqueue_active()??
>> >> >> <snip>
>> >> >> 
>> >> >> There are around 200 occurrences of waitqueue_active() in the kernel
>> >> >> source, and most of the places which use it before wake_up are either
>> >> >> protected by some spin lock, or already has a memory barrier or some
>> >> >> kind of atomic operation before it.
>> >> >> 
>> >> >> Simply adding smp_mb() to waitqueue_active() would incur extra cost in
>> >> >> many cases and won't be a good idea.
>> >> >> 
>> >> >> Another way to solve this problem is to remove the waitqueue_active(),
>> >> >> making the code look like this;
>> >> >> 	if (wq)
>> >> >> 		wake_up_interruptible(wq);
>> >> >> This also fixes the problem because the spinlock in the wake_up*() acts
>> >> >> as a memory barrier and prevents the code from being reordered by the
>> >> >> CPU (and it also makes the resulting code is much simpler).
>> >> > 
>> >> > I might not care which we did, except I don't have the means to test
>> >> > this quickly, and I guess this is some of our most frequently called
>> >> > code.
>> >> > 
>> >> > I suppose your patch is the most conservative approach, as the
>> >> > alternative is a spinlock/unlock in wake_up_interruptible, which I
>> >> > assume is necessarily more expensive than an smp_mb().
>> >> > 
>> >> > As far as I can tell it's been this way since forever.  (Well, since a
>> >> > 2002 patch "NFSD: TCP: rationalise locking in RPC server routines" which
>> >> > removed some spinlocks from the data_ready routines.)
>> >> > 
>> >> > I don't understand what the actual race is yet (which code exactly is
>> >> > missing the wakeup in this case?  nfsd threads seem to instead get
>> >> > woken up by the wake_up_process() in svc_xprt_do_enqueue().)
>> >> 
>> >> Thank you for the reply.  I tried looking into this.
>> >> 
>> >> The callbacks in net/sunrpc/svcsock.c are set up in svc_tcp_init() and
>> >> svc_udp_init(), which are both called from svc_setup_socket().
>> >> svc_setup_socket() is called (indirectly) from lockd, nfsd, and nfsv4
>> >> callback port related code.
>> >> 
>> >> Maybe I'm wrong, but there might not be any kernel code that is using
>> >> the socket's wait queue in this case.
>> > 
>> > As Trond points out there are probably waiters internal to the
>> > networking code.
>> 
>> Trond and Bruce, thank you for the comment.  I was able to find the call
>> to the wait function that was called from nfsd.
>> 
>> sk_stream_wait_connect() and sk_stream_wait_memory() were called from
>> either do_tcp_sendpages() or tcp_sendmsg() called from within
>> svc_send().  sk_stream_wait_connect() shouldn't be called at this point,
>> because the socket has already been used to receive the rpc request.
>> 
>> On the wake_up side, sk_write_space() is called from the following
>> locations.  The relevant ones seems to be preceded by atomic_sub or a
>> memory barrier.
>> + ksocknal_write_space [drivers/staging/lustre/lnet/klnds/socklnd/socklnd_lib.c:633]
>> + atm_pop_raw [net/atm/raw.c:40]
>> + sock_setsockopt [net/core/sock.c:740]
>> + sock_wfree [net/core/sock.c:1630]
>>   Preceded by atomic_sub in sock_wfree()
>> + ccid3_hc_tx_packet_recv [net/dccp/ccids/ccid3.c:442]
>> + do_tcp_sendpages [net/ipv4/tcp.c:1008]
>> + tcp_sendmsg [net/ipv4/tcp.c:1300]
>> + do_tcp_setsockopt [net/ipv4/tcp.c:2597]
>> + tcp_new_space [net/ipv4/tcp_input.c:4885]
>>   Preceded by smp_mb__after_atomic in tcp_check_space()
>> + llc_conn_state_process [net/llc/llc_conn.c:148]
>> + pipe_rcv_status [net/phonet/pep.c:312]
>> + pipe_do_rcv [net/phonet/pep.c:440]
>> + pipe_start_flow_control [net/phonet/pep.c:554]
>> + svc_sock_setbufsize [net/sunrpc/svcsock.c:45]
>> 
>> sk_state_change() calls related to TCP/IP were called from the following
>> places.
>> + inet_shutdown [net/ipv4/af_inet.c:825]
>>   This shouldn't be called when waiting
>> + tcp_done [net/ipv4/tcp.c:3078]
>>   spin_lock*/spin_unlock* is called in lock_timer_base
>> + tcp_fin [net/ipv4/tcp_input.c:4031]
>>   atomic_long_sub is called from sk_memory_allocated_sub called within
>>   sk_mem_reclaim
>> + tcp_finish_connect [net/ipv4/tcp_input.c:5415]
>>   This shoudn't be called when waiting
>> + tcp_rcv_state_process [net/ipv4/tcp_input.c:5807,5880]
>>   The socket shouldn't be in TCP_SYN_RECV nor TCP_FIN_WAIT1 states when
>>   waiting
>> 
>> I think the wait queue won't be used for being woken up by
>> svc_{tcp,udp}_data_ready, because nfsd doesn't read from a socket.
> 
> Looking, well, I guess kernel_recvmsg() does read from a socket, but I
> assume calling with MSG_DONTWAIT means that it doesn't block.
> 
>> So with the current implementation, it seems there shouldn't be any
>> problems even if the memory barrier is missing.
> 
> Thanks for the detailed investigation.
> 
> I think it would be worth adding a comment if that might help someone
> having to reinvestigate this again some day.

It would be nice, but I find it difficult to write a comment in the
sunrpc layer why a memory barrier isn't necessary, using the knowledge
of how nfsd uses it, and the current implementation of the network code.

Personally, I would prefer removing the call to waitqueue_active() which
would make the memory barrier totally unnecessary at the cost of a
spin_lock + spin_unlock by unconditionally calling
wake_up_interruptible.
---
Kosuke TATSUKAWA  | 3rd IT Platform Department
                  | IT Platform Division, NEC Corporation
                  | tatsu@xxxxxxxxxxxxx
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