> On Apr 1, 2020, at 5:55 PM, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote:
>
> On Wed, 2020-04-01 at 15:37 -0400, Chuck Lever wrote:
>> If an RPC task is signaled while it is running and the transport is
>> not connected, it will never sleep and never be terminated. This can
>> happen when a RPC transport is shut down: the remaining tasks are
>> signalled, but the transport is disconnected.
>>
>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>> ---
>> net/sunrpc/sched.c | 14 ++++++--------
>> 1 file changed, 6 insertions(+), 8 deletions(-)
>>
>> Interested in comments and suggestions.
>>
>> Nearly every time my NFS/RDMA client unmounts when using krb5, the
>> umount hangs (killably). I tracked it down to an NFSv3 NULL request
>> that is signalled but loops and does not exit.
>>
>>
>> diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
>> index 55e900255b0c..905c31f75593 100644
>> --- a/net/sunrpc/sched.c
>> +++ b/net/sunrpc/sched.c
>> @@ -905,6 +905,12 @@ static void __rpc_execute(struct rpc_task *task)
>> trace_rpc_task_run_action(task, do_action);
>> do_action(task);
>>
>> + if (RPC_SIGNALLED(task)) {
>> + task->tk_rpc_status = -ERESTARTSYS;
>> + rpc_exit(task, -ERESTARTSYS);
>> + break;
>> + }
>> +
>
> Hmm... I'd really prefer to avoid this kind of check in the tight loop.
Certainly performance is a concern, but compared to the indirect
function call directly preceding this check, it is small potatoes.
Adding an "unlikely()" to the RPC_SIGNALLED() macro might help
mitigate the execution pipeline bubble it would cause.
> Why is this NULL request looping?
Here's an ftrace log I captured a couple of days ago.
umount.nfs-7540 [004] 938.823064: rpc_task_begin: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=ACTIVE status=0 action=(nil)s
kworker/u25:0-6092 [000] 938.823073: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_start
kworker/u25:0-6092 [000] 938.823075: rpc_request: task:18647@1 nfsv3 NULL (async)
kworker/u25:0-6092 [000] 938.823076: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_reserve
kworker/u25:0-6092 [000] 938.823078: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_reserveresult
kworker/u25:0-6092 [000] 938.823079: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_refresh
kworker/u25:0-6092 [000] 938.823080: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_refreshresult
kworker/u25:0-6092 [000] 938.823081: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_allocate
kworker/u25:0-6092 [000] 938.823084: xprtrdma_op_allocate: task:18647@1 req=0xffff8884691ca000 (1624, 60)
kworker/u25:0-6092 [000] 938.823085: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE status=0 action=call_encode
kworker/u25:0-6092 [000] 938.823088: rpcgss_seqno: task:18647@1 xid=0xfd7e5fb8 seqno=21981
kworker/u25:0-6092 [000] 938.823117: xprt_enq_xmit: task:18647@1 xid=0xfd7e5fb8 seqno=21981 stage=4
kworker/u25:0-6092 [000] 938.823118: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_XMIT|NEED_RECV status=0 action=call_transmit
umount.nfs-7540 [004] 938.823119: rpc_task_signalled: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_XMIT|NEED_RECV status=0 action=call_transmit_status
kworker/u25:0-6092 [000] 938.823119: xprt_reserve_cong: task:18647@1 snd_task:18647 cong=0 cwnd=16384
kworker/u25:0-6092 [000] 938.823121: xprt_transmit: task:18647@1 xid=0xfd7e5fb8 seqno=21981 status=-512
kworker/u25:0-6092 [000] 938.823127: xprt_release_cong: task:18647@1 snd_task:4294967295 cong=0 cwnd=16384
kworker/u25:0-6092 [000] 938.823127: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=0 action=call_transmit_status
kworker/u25:0-6092 [000] 938.823129: rpc_task_sleep: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=0 timeout=0 queue=xprt_pending
kworker/u25:0-6092 [000] 938.823132: rpc_task_wakeup: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|QUEUED|ACTIVE|NEED_RECV|SIGNALLED status=-107 timeout=60000 queue=xprt_pending
kworker/u25:0-6092 [000] 938.823133: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=-107 action=xprt_timer
kworker/u25:0-6092 [000] 938.823133: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=-107 action=call_status
kworker/u25:0-6092 [000] 938.823136: rpc_call_status: task:18647@1 status=-107
kworker/u25:0-6092 [000] 938.823137: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=0 action=call_encode
kworker/u25:0-6092 [000] 938.823138: rpcgss_seqno: task:18647@1 xid=0xfd7e5fb8 seqno=21982
kworker/u25:0-6092 [000] 938.823146: xprt_enq_xmit: task:18647@1 xid=0xfd7e5fb8 seqno=21982 stage=4
kworker/u25:0-6092 [000] 938.823147: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_XMIT|NEED_RECV|SIGNALLED status=0 action=call_transmit
kworker/u25:0-6092 [000] 938.823147: xprt_reserve_cong: task:18647@1 snd_task:18647 cong=0 cwnd=16384
kworker/u25:0-6092 [000] 938.823148: xprt_transmit: task:18647@1 xid=0xfd7e5fb8 seqno=21982 status=-512
kworker/u25:0-6092 [000] 938.823149: xprt_release_cong: task:18647@1 snd_task:4294967295 cong=0 cwnd=16384
kworker/u25:0-6092 [000] 938.823149: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=0 action=call_transmit_status
kworker/u25:0-6092 [000] 938.823150: rpc_task_sleep: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=0 timeout=0 queue=xprt_pending
kworker/u25:0-6092 [000] 938.823151: rpc_task_wakeup: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|QUEUED|ACTIVE|NEED_RECV|SIGNALLED status=-107 timeout=120000 queue=xprt_pending
kworker/u25:0-6092 [000] 938.823152: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=-107 action=xprt_timer
kworker/u25:0-6092 [000] 938.823152: rpc_task_run_action: task:18647@1 flags=ASYNC|DYNAMIC|SOFT runstate=RUNNING|ACTIVE|NEED_RECV|SIGNALLED status=-107 action=call_status
kworker/u25:0-6092 [000] 938.823153: rpc_call_status: task:18647@1 status=-107
And here, the looping continues.
My analysis is that the signal occurs while the task is running/active
in a section of code that continues to iterate on reconnecting without
checking for a signal. Note that the tail of xprt_transmit will not set
tk_status in this case, so it remains set to zero upon entry to
call_transmit_status.
I like the scheduler fix because I don't believe we can easily tell if
there might be other such critical sections in the client's state
machine where a loop might occur with no exit condition based on a
signal.
This fix closes the door on all such bugs.
--
Chuck Lever
