RHEL6.6 NFS クライアントで、NFS マウントの書き込み負荷が低いときに 'ls' がハングアップする

Issue

PID: 49032  TASK: ffff880108f53540  CPU: 23  COMMAND: "ls"
 #0 [ffff88010481d388] schedule at ffffffff81529970
 #1 [ffff88010481d460] schedule_timeout at ffffffff8152a8c5
 #2 [ffff88010481d510] wait_for_common at ffffffff8152a543
 #3 [ffff88010481d5a0] wait_for_completion at ffffffff8152a65d
 #4 [ffff88010481d5b0] call_usermodehelper_exec at ffffffff81096e2c
 #5 [ffff88010481d600] call_sbin_request_key at ffffffff8122af1e
 #6 [ffff88010481d760] request_key_and_link at ffffffff8122aa55
 #7 [ffff88010481d800] request_key at ffffffff8122ac20
 #8 [ffff88010481d850] nfs_idmap_request_key at ffffffffa036fbe5 [nfs]
 #9 [ffff88010481d8b0] nfs_idmap_lookup_id at ffffffffa0370214 [nfs]
#10 [ffff88010481d900] nfs_map_name_to_uid at ffffffffa0370655 [nfs]
#11 [ffff88010481d930] decode_getfattr_attrs at ffffffffa036a634 [nfs]
#12 [ffff88010481d9c0] decode_getfattr_generic.clone.0 at ffffffffa036a6f3 [nfs]
#13 [ffff88010481da10] nfs4_xdr_dec_getattr at ffffffffa036aeaf [nfs]
#14 [ffff88010481dab0] rpcauth_unwrap_resp at ffffffffa02b7274 [sunrpc]
#15 [ffff88010481db00] call_decode at ffffffffa02ab9b9 [sunrpc]
#16 [ffff88010481db60] __rpc_execute at ffffffffa02b58e7 [sunrpc]
#17 [ffff88010481dbd0] rpc_execute at ffffffffa02b5c21 [sunrpc]
#18 [ffff88010481dc00] rpc_run_task at ffffffffa02ac465 [sunrpc]
#19 [ffff88010481dc20] rpc_call_sync at ffffffffa02ac582 [sunrpc]
#20 [ffff88010481dc80] _nfs4_call_sync at ffffffffa035eace [nfs]
#21 [ffff88010481dc90] _nfs4_proc_getattr at ffffffffa035735c [nfs]
#22 [ffff88010481dd40] nfs4_proc_getattr at ffffffffa0359ff6 [nfs]
#23 [ffff88010481dd90] __nfs_revalidate_inode at ffffffffa0340073 [nfs]
#24 [ffff88010481ddd0] nfs_revalidate_inode at ffffffffa0340da6 [nfs]
#25 [ffff88010481de00] nfs_getattr at ffffffffa0340e86 [nfs]
#26 [ffff88010481de40] vfs_getattr at ffffffff81193b41
#27 [ffff88010481de80] vfs_fstatat at ffffffff81193bd4
#28 [ffff88010481dee0] vfs_lstat at ffffffff81193c7e
#29 [ffff88010481def0] sys_newlstat at ffffffff81193ca4
#30 [ffff88010481df80] system_call_fastpath at ffffffff8100b072
    RIP: 00000033b02dae95  RSP: 00007fff7e8a2cf8  RFLAGS: 00010246
    RAX: 0000000000000006  RBX: ffffffff8100b072  RCX: 0000000000000000
    RDX: 0000000001d53dd0  RSI: 0000000001d53dd0  RDI: 00007fff7e8a2d10
    RBP: 00007fff7e8a30e0   R8: 0000000001d58280   R9: 0000000000000001
    R10: 000000000000000f  R11: 0000000000000246  R12: 0000000001d58433
    R13: 0000000001d53dc0  R14: 0000000000000000  R15: 0000000001d58433
    ORIG_RAX: 0000000000000006  CS: 0033  SS: 002b