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. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
