Heh, look what it does...


4======================================================
4WARNING: possible circular locking dependency detected
4.13.0-rc2-00317-gadc6764a3adf-dirty #797 Tainted: G        W      
4------------------------------------------------------
4startpar/582 is trying to acquire lock:
c (c(complete)&barr->donec){+.+.}c, at: [<ffffffff8110de4d>] 
flush_work+0x1fd/0x2c0
4
but task is already holding lock:
c (clockc#3c){+.+.}c, at: [<ffffffff8122e866>] 
lru_add_drain_all_cpuslocked+0x46/0x1a0
4
which lock already depends on the new lock.

4
the existing dependency chain (in reverse order) is:

-> #4c (clockc#3c){+.+.}c:
       __lock_acquire+0x10a5/0x1100
       lock_acquire+0xea/0x1f0
       __mutex_lock+0x6c/0x960
       mutex_lock_nested+0x1b/0x20
       lru_add_drain_all_cpuslocked+0x46/0x1a0
       lru_add_drain_all+0x13/0x20
       SyS_mlockall+0xb8/0x1c0
       entry_SYSCALL_64_fastpath+0x23/0xc2

-> #3c (ccpu_hotplug_lock.rw_semc){++++}c:
       __lock_acquire+0x10a5/0x1100
       lock_acquire+0xea/0x1f0
       cpus_read_lock+0x2a/0x90
       kmem_cache_create+0x2a/0x1d0
       scsi_init_sense_cache+0xa0/0xc0
       scsi_add_host_with_dma+0x67/0x360
       isci_pci_probe+0x873/0xc90
       local_pci_probe+0x42/0xa0
       work_for_cpu_fn+0x14/0x20
       process_one_work+0x273/0x6b0
       worker_thread+0x21b/0x3f0
       kthread+0x147/0x180
       ret_from_fork+0x2a/0x40

-> #2c (cscsi_sense_cache_mutexc){+.+.}c:
       __lock_acquire+0x10a5/0x1100
       lock_acquire+0xea/0x1f0
       __mutex_lock+0x6c/0x960
       mutex_lock_nested+0x1b/0x20
       scsi_init_sense_cache+0x3d/0xc0
       scsi_add_host_with_dma+0x67/0x360
       isci_pci_probe+0x873/0xc90
       local_pci_probe+0x42/0xa0
       work_for_cpu_fn+0x14/0x20
       process_one_work+0x273/0x6b0
       worker_thread+0x21b/0x3f0
       kthread+0x147/0x180
       ret_from_fork+0x2a/0x40

-> #1c (c(&wfc.work)c){+.+.}c:
       process_one_work+0x244/0x6b0
       worker_thread+0x21b/0x3f0
       kthread+0x147/0x180
       ret_from_fork+0x2a/0x40
       0xffffffffffffffff

-> #0c (c(complete)&barr->donec){+.+.}c:
       check_prev_add+0x3be/0x700
       __lock_acquire+0x10a5/0x1100
       lock_acquire+0xea/0x1f0
       wait_for_completion+0x3b/0x130
       flush_work+0x1fd/0x2c0
       lru_add_drain_all_cpuslocked+0x158/0x1a0
       lru_add_drain_all+0x13/0x20
       SyS_mlockall+0xb8/0x1c0
       entry_SYSCALL_64_fastpath+0x23/0xc2

other info that might help us debug this:

Chain exists of:
  c(complete)&barr->donec --> ccpu_hotplug_lock.rw_semc --> clockc#3c

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(clockc#3c);
                               lock(ccpu_hotplug_lock.rw_semc);
                               lock(clockc#3c);
  lock(c(complete)&barr->donec);

 *** DEADLOCK ***

2 locks held by startpar/582:
 #0: c (ccpu_hotplug_lock.rw_semc){++++}c, at: [<ffffffff8122e9ce>] 
lru_add_drain_all+0xe/0x20
 #1: c (clockc#3c){+.+.}c, at: [<ffffffff8122e866>] 
lru_add_drain_all_cpuslocked+0x46/0x1a0

stack backtrace:
dCPU: 23 PID: 582 Comm: startpar Tainted: G        W       
4.13.0-rc2-00317-gadc6764a3adf-dirty #797
dHardware name: Intel Corporation S2600GZ/S2600GZ, BIOS 
SE5C600.86B.02.02.0002.122320131210 12/23/2013
dCall Trace:
d dump_stack+0x86/0xcf
d print_circular_bug+0x203/0x2f0
d check_prev_add+0x3be/0x700
d ? add_lock_to_list.isra.30+0xc0/0xc0
d ? is_bpf_text_address+0x82/0xe0
d ? unwind_get_return_address+0x1f/0x30
d __lock_acquire+0x10a5/0x1100
d ? __lock_acquire+0x10a5/0x1100
d ? add_lock_to_list.isra.30+0xc0/0xc0
d lock_acquire+0xea/0x1f0
d ? flush_work+0x1fd/0x2c0
d wait_for_completion+0x3b/0x130
d ? flush_work+0x1fd/0x2c0
d flush_work+0x1fd/0x2c0
d ? flush_workqueue_prep_pwqs+0x1c0/0x1c0
d ? trace_hardirqs_on+0xd/0x10
d lru_add_drain_all_cpuslocked+0x158/0x1a0
d lru_add_drain_all+0x13/0x20
d SyS_mlockall+0xb8/0x1c0
d entry_SYSCALL_64_fastpath+0x23/0xc2
dRIP: 0033:0x7f818d2e54c7
dRSP: 002b:00007fffcce83798 EFLAGS: 00000246c ORIG_RAX: 0000000000000097
dRAX: ffffffffffffffda RBX: 0000000000000046 RCX: 00007f818d2e54c7
dRDX: 0000000000000000 RSI: 00007fffcce83650 RDI: 0000000000000003
dRBP: 000000000002c010 R08: 0000000000000000 R09: 0000000000000000
dR10: 0000000000000008 R11: 0000000000000246 R12: 000000000002d000
dR13: 000000000002c010 R14: 0000000000001000 R15: 00007f818d599b00

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