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 -- 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
