do_iter_write lockdep trace

[Dave Jones]

>From current master.

[  296.457954] ============================================
[  296.458081] WARNING: possible recursive locking detected
[  296.458201] 4.12.0-think+ #7 Not tainted
[  296.458291] --------------------------------------------
[  296.458409] trinity-c31/1551 is trying to acquire lock:
[  296.458556]  (
[  296.458603] sb_writers
[  296.458656] #4
[  296.458694] ){.+.+.+}
[  296.458749] , at: [<ffffffff81267074>] do_iter_write+0x134/0x220
[  296.458885] 
               but task is already holding lock:
[  296.459020]  (
[  296.459059] sb_writers
[  296.459111] #4
[  296.459149] ){.+.+.+}
[  296.459202] , at: [<ffffffff812679f6>] do_sendfile+0x366/0x3c0
[  296.459333] 
               other info that might help us debug this:
[  296.459485]  Possible unsafe locking scenario:

[  296.459631]        CPU0
[  296.459685]        ----
[  296.459739]   lock(
[  296.459785] sb_writers
[  296.459840] #4
[  296.459876] );
[  296.459913]   lock(
[  296.459961] sb_writers
[  296.460021] #4
[  296.460060] );
[  296.460097] 
                *** DEADLOCK ***

[  296.460220]  May be due to missing lock nesting notation

[  296.460367] 1 lock held by trinity-c31/1551:
[  296.460463]  #0: 
[  296.460507]  (
[  296.460544] sb_writers
[  296.460599] #4
[  296.460637] ){.+.+.+}
[  296.460689] , at: [<ffffffff812679f6>] do_sendfile+0x366/0x3c0
[  296.460820] 
               stack backtrace:
[  296.460916] CPU: 3 PID: 1551 Comm: trinity-c31 Not tainted 4.12.0-think+ #7 
[  296.461188] Call Trace:
[  296.461246]  dump_stack+0x68/0x9f
[  296.461322]  __lock_acquire+0x833/0x1360
[  296.461412]  ? iter_file_splice_write+0x8c/0x420
[  296.461517]  ? init_object+0x69/0xa0
[  296.461599]  lock_acquire+0xe7/0x1e0
[  296.461680]  ? lock_acquire+0xe7/0x1e0
[  296.461765]  ? do_iter_write+0x134/0x220
[  296.461855]  __sb_start_write+0xcb/0x1e0
[  296.461944]  ? do_iter_write+0x134/0x220
[  296.462043]  ? iter_file_splice_write+0x8c/0x420
[  296.462148]  do_iter_write+0x134/0x220
[  296.462233]  vfs_iter_write+0x19/0x30
[  296.462317]  iter_file_splice_write+0x28d/0x420
[  296.462424]  direct_splice_actor+0x36/0x40
[  296.462517]  splice_direct_to_actor+0xe5/0x220
[  296.462618]  ? generic_pipe_buf_nosteal+0x10/0x10
[  296.462725]  do_splice_direct+0x9e/0xd0
[  296.462812]  do_sendfile+0x1d7/0x3c0
[  296.462893]  SyS_sendfile64+0x73/0xe0
[  296.462978]  do_syscall_64+0x66/0x190
[  296.463072]  entry_SYSCALL64_slow_path+0x25/0x25
[  296.463178] RIP: 0033:0x7fbe852001e9
[  296.463260] RSP: 002b:00007ffe16f8b328 EFLAGS: 00000246
[  296.470299]  ORIG_RAX: 0000000000000028
[  296.477309] RAX: ffffffffffffffda RBX: 0000000000000028 RCX: 00007fbe852001e9
[  296.484450] RDX: 00007fbe85714000 RSI: 000000000000011c RDI: 000000000000011c
[  296.491497] RBP: 00007ffe16f8b3d0 R08: 0000884b02130845 R09: fffffffffffffffd
[  296.498503] R10: 0000000000001000 R11: 0000000000000246 R12: 0000000000000002
[  296.505525] R13: 00007fbe857f7058 R14: 00007fbe858d7698 R15: 00007fbe857f7000