On Tue, Aug 15, 2017 at 10:20:20AM +0200, Ingo Molnar wrote:
>
> So with the latest fixes there's a new lockdep warning on one of my testboxes:
>
> [ 11.322487] EXT4-fs (sda2): mounted filesystem with ordered data mode. Opts: (null)
>
> [ 11.495661] ======================================================
> [ 11.502093] WARNING: possible circular locking dependency detected
> [ 11.508507] 4.13.0-rc5-00497-g73135c58-dirty #1 Not tainted
> [ 11.514313] ------------------------------------------------------
> [ 11.520725] umount/533 is trying to acquire lock:
> [ 11.525657] ((complete)&barr->done){+.+.}, at: [<ffffffff810fdbb3>] flush_work+0x213/0x2f0
> [ 11.534411]
> but task is already holding lock:
> [ 11.540661] (lock#3){+.+.}, at: [<ffffffff8122678d>] lru_add_drain_all_cpuslocked+0x3d/0x190
> [ 11.549613]
> which lock already depends on the new lock.
>
> The full splat is below. The kernel config is nothing fancy - distro derived,
> pretty close to defconfig, with lockdep enabled.
I see...
Worker A : acquired of wfc.work -> wait for cpu_hotplug_lock to be released
Task B : acquired of cpu_hotplug_lock -> wait for lock#3 to be released
Task C : acquired of lock#3 -> wait for completion of barr->done
Worker D : wait for wfc.work to be released -> will complete barr->done
The report below is telling that a deadlock would happen if the four tasks
run simultaniously. Here, I wonder if wfc.work sould be acquired with a
write version. I am not familiar with workqueue. Could anyone explain it
for me?
Thank you,
Byungchul
> Thanks,
>
> Ingo
>
> [ 11.322487] EXT4-fs (sda2): mounted filesystem with ordered data mode. Opts: (null)
>
> [ 11.495661] ======================================================
> [ 11.502093] WARNING: possible circular locking dependency detected
> [ 11.508507] 4.13.0-rc5-00497-g73135c58-dirty #1 Not tainted
> [ 11.514313] ------------------------------------------------------
> [ 11.520725] umount/533 is trying to acquire lock:
> [ 11.525657] ((complete)&barr->done){+.+.}, at: [<ffffffff810fdbb3>] flush_work+0x213/0x2f0
> [ 11.534411]
> but task is already holding lock:
> [ 11.540661] (lock#3){+.+.}, at: [<ffffffff8122678d>] lru_add_drain_all_cpuslocked+0x3d/0x190
> [ 11.549613]
> which lock already depends on the new lock.
>
> [ 11.558349]
> the existing dependency chain (in reverse order) is:
> [ 11.566229]
> -> #3 (lock#3){+.+.}:
> [ 11.571439] lock_acquire+0xe7/0x1d0
> [ 11.575765] __mutex_lock+0x75/0x8e0
> [ 11.580086] lru_add_drain_all_cpuslocked+0x3d/0x190
> [ 11.585797] lru_add_drain_all+0xf/0x20
> [ 11.590402] invalidate_bdev+0x3e/0x60
> [ 11.594901] ext4_put_super+0x1f9/0x3d0
> [ 11.599485] generic_shutdown_super+0x64/0x110
> [ 11.604685] kill_block_super+0x21/0x50
> [ 11.609270] deactivate_locked_super+0x39/0x70
> [ 11.614462] cleanup_mnt+0x3b/0x70
> [ 11.618612] task_work_run+0x72/0x90
> [ 11.622955] exit_to_usermode_loop+0x93/0xa0
> [ 11.627971] do_syscall_64+0x1a2/0x1c0
> [ 11.632470] return_from_SYSCALL_64+0x0/0x7a
> [ 11.637487]
> -> #2 (cpu_hotplug_lock.rw_sem){++++}:
> [ 11.644144] lock_acquire+0xe7/0x1d0
> [ 11.648487] cpus_read_lock+0x2b/0x60
> [ 11.652897] apply_workqueue_attrs+0x12/0x50
> [ 11.657917] __alloc_workqueue_key+0x2f2/0x510
> [ 11.663110] scsi_host_alloc+0x353/0x470
> [ 11.667780] _scsih_probe+0x5bb/0x7b0
> [ 11.672192] local_pci_probe+0x3f/0x90
> [ 11.676714] work_for_cpu_fn+0x10/0x20
> [ 11.681213] process_one_work+0x1fc/0x670
> [ 11.685971] worker_thread+0x219/0x3e0
> [ 11.690469] kthread+0x13a/0x170
> [ 11.694465] ret_from_fork+0x27/0x40
> [ 11.698790]
> -> #1 ((&wfc.work)){+.+.}:
> [ 11.704433] worker_thread+0x219/0x3e0
> [ 11.708930] kthread+0x13a/0x170
> [ 11.712908] ret_from_fork+0x27/0x40
> [ 11.717234] 0xffffffffffffffff
> [ 11.721142]
> -> #0 ((complete)&barr->done){+.+.}:
> [ 11.727633] __lock_acquire+0x1433/0x14a0
> [ 11.732392] lock_acquire+0xe7/0x1d0
> [ 11.736715] wait_for_completion+0x4e/0x170
> [ 11.741664] flush_work+0x213/0x2f0
> [ 11.745919] lru_add_drain_all_cpuslocked+0x149/0x190
> [ 11.751718] lru_add_drain_all+0xf/0x20
> [ 11.756303] invalidate_bdev+0x3e/0x60
> [ 11.760819] ext4_put_super+0x1f9/0x3d0
> [ 11.765403] generic_shutdown_super+0x64/0x110
> [ 11.770596] kill_block_super+0x21/0x50
> [ 11.775181] deactivate_locked_super+0x39/0x70
> [ 11.780372] cleanup_mnt+0x3b/0x70
> [ 11.784522] task_work_run+0x72/0x90
> [ 11.788848] exit_to_usermode_loop+0x93/0xa0
> [ 11.793875] do_syscall_64+0x1a2/0x1c0
> [ 11.798399] return_from_SYSCALL_64+0x0/0x7a
> [ 11.803416]
> other info that might help us debug this:
>
> [ 11.811997] Chain exists of:
> (complete)&barr->done --> cpu_hotplug_lock.rw_sem --> lock#3
>
> [ 11.823810] Possible unsafe locking scenario:
>
> [ 11.830120] CPU0 CPU1
> [ 11.834878] ---- ----
> [ 11.839636] lock(lock#3);
> [ 11.842653] lock(cpu_hotplug_lock.rw_sem);
> [ 11.849697] lock(lock#3);
> [ 11.855236] lock((complete)&barr->done);
> [ 11.859560]
> *** DEADLOCK ***
>
> [ 11.866054] 3 locks held by umount/533:
> [ 11.870117] #0: (&type->s_umount_key#24){+.+.}, at: [<ffffffff8129b7ad>] deactivate_super+0x4d/0x60
> [ 11.879737] #1: (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffff812268ea>] lru_add_drain_all+0xa/0x20
> [ 11.889445] #2: (lock#3){+.+.}, at: [<ffffffff8122678d>] lru_add_drain_all_cpuslocked+0x3d/0x190
> [ 11.898805]
> stack backtrace:
> [ 11.903573] CPU: 12 PID: 533 Comm: umount Not tainted 4.13.0-rc5-00497-g73135c58-dirty #1
> [ 11.912169] Hardware name: Supermicro H8DG6/H8DGi/H8DG6/H8DGi, BIOS 2.0b 03/01/2012
> [ 11.920759] Call Trace:
> [ 11.923433] dump_stack+0x5e/0x8e
> [ 11.926975] print_circular_bug+0x204/0x310
> [ 11.931385] ? add_lock_to_list.isra.29+0xb0/0xb0
> [ 11.936316] check_prev_add+0x444/0x860
> [ 11.940382] ? generic_shutdown_super+0x64/0x110
> [ 11.945237] ? add_lock_to_list.isra.29+0xb0/0xb0
> [ 11.950168] ? __lock_acquire+0x1433/0x14a0
> [ 11.954578] __lock_acquire+0x1433/0x14a0
> [ 11.958818] lock_acquire+0xe7/0x1d0
> [ 11.962621] ? flush_work+0x213/0x2f0
> [ 11.966506] wait_for_completion+0x4e/0x170
> [ 11.970915] ? flush_work+0x213/0x2f0
> [ 11.974807] ? flush_work+0x1e6/0x2f0
> [ 11.978699] flush_work+0x213/0x2f0
> [ 11.982416] ? flush_workqueue_prep_pwqs+0x1b0/0x1b0
> [ 11.987610] ? mark_held_locks+0x66/0x90
> [ 11.991778] ? queue_work_on+0x41/0x70
> [ 11.995755] lru_add_drain_all_cpuslocked+0x149/0x190
> [ 12.001034] lru_add_drain_all+0xf/0x20
> [ 12.005124] invalidate_bdev+0x3e/0x60
> [ 12.009094] ext4_put_super+0x1f9/0x3d0
> [ 12.013159] generic_shutdown_super+0x64/0x110
> [ 12.017856] kill_block_super+0x21/0x50
> [ 12.021922] deactivate_locked_super+0x39/0x70
> [ 12.026591] cleanup_mnt+0x3b/0x70
> [ 12.030242] task_work_run+0x72/0x90
> [ 12.034063] exit_to_usermode_loop+0x93/0xa0
> [ 12.038561] do_syscall_64+0x1a2/0x1c0
> [ 12.042541] entry_SYSCALL64_slow_path+0x25/0x25
> [ 12.047384] RIP: 0033:0x7fc3f2854a37
> [ 12.051189] RSP: 002b:00007fff660582b8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
> [ 12.059162] RAX: 0000000000000000 RBX: 00000074471c14e0 RCX: 00007fc3f2854a37
> [ 12.066530] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000074471c22e0
> [ 12.073895] RBP: 00000074471c22e0 R08: 0000000000000000 R09: 0000000000000002
> [ 12.081264] R10: 00007fff66058050 R11: 0000000000000246 R12: 00007fc3f35e6890
> [ 12.088656] R13: 0000000000000000 R14: 00000074471c1660 R15: 0000000000000000
> [ 12.110307] dracut: Checking ext4: /dev/sda2
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