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

[Kosuke Tatsukawa]

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.

So with the current implementation, it seems there shouldn't be any
problems even if the memory barrier is missing.
---
Kosuke TATSUKAWA  | 3rd IT Platform Department
                  | IT Platform Division, NEC Corporation
                  | tatsu@xxxxxxxxxxxxx
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