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Date:   Sat, 12 Mar 2022 01:53:26 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Byungchul Park <byungchul.park@lge.com>
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Subject: Re: [PATCH v4 00/24] DEPT(Dependency Tracker)
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On Fri, Mar 04, 2022 at 04:06:19PM +0900, Byungchul Park wrote:
> Hi Linus and folks,
> 
> I've been developing a tool for detecting deadlock possibilities by
> tracking wait/event rather than lock(?) acquisition order to try to
> cover all synchonization machanisms. It's done on v5.17-rc1 tag.
> 
> https://github.com/lgebyungchulpark/linux-dept/commits/dept1.14_on_v5.17-rc1
>

Small feedback unrelated to thread:
I'm not sure "Need to expand the ring buffer" is something to call
WARN(). Is this stack trace useful for something?
========

Hello Byungchul. These are two warnings of DEPT on system.
Both cases look similar.

In what case DEPT says (unknown)?
I'm not sure we can properly debug this.

===================================================
DEPT: Circular dependency has been detected.
5.17.0-rc1+ #3 Tainted: G        W        
---------------------------------------------------
summary
---------------------------------------------------
*** AA DEADLOCK ***

context A
    [S] (unknown)(&vfork:0)
    [W] wait_for_completion_killable(&vfork:0)
    [E] complete(&vfork:0)

[S]: start of the event context
[W]: the wait blocked
[E]: the event not reachable
---------------------------------------------------
context A's detail
---------------------------------------------------
context A
    [S] (unknown)(&vfork:0)
    [W] wait_for_completion_killable(&vfork:0)
    [E] complete(&vfork:0)

[S] (unknown)(&vfork:0):
(N/A)

[W] wait_for_completion_killable(&vfork:0):
[<ffffffc00802204c>] kernel_clone+0x25c/0x2b8
stacktrace:
      dept_wait+0x74/0x88
      wait_for_completion_killable+0x60/0xa0
      kernel_clone+0x25c/0x2b8
      __do_sys_clone+0x5c/0x74
      __arm64_sys_clone+0x18/0x20
      invoke_syscall.constprop.0+0x78/0xc4
      do_el0_svc+0x98/0xd0
      el0_svc+0x44/0xe4
      el0t_64_sync_handler+0xb0/0x12c
      el0t_64_sync+0x158/0x15c

[E] complete(&vfork:0):
[<ffffffc00801f49c>] mm_release+0x7c/0x90
stacktrace:
      dept_event+0xe0/0x100
      complete+0x48/0x98
      mm_release+0x7c/0x90
      exit_mm_release+0xc/0x14
      do_exit+0x1b4/0x81c
      do_group_exit+0x30/0x9c
      __wake_up_parent+0x0/0x24
      invoke_syscall.constprop.0+0x78/0xc4
      do_el0_svc+0x98/0xd0
      el0_svc+0x44/0xe4
      el0t_64_sync_handler+0xb0/0x12c
      el0t_64_sync+0x158/0x15c
---------------------------------------------------
information that might be helpful
---------------------------------------------------
CPU: 6 PID: 229 Comm: start-stop-daem Tainted: G        W         5.17.0-rc1+ #3
Hardware name: linux,dummy-virt (DT)
Call trace:
 dump_backtrace.part.0+0x9c/0xc4
 show_stack+0x14/0x28
 dump_stack_lvl+0x9c/0xcc
 dump_stack+0x14/0x2c
 print_circle+0x2d4/0x438
 cb_check_dl+0x44/0x70
 bfs+0x60/0x168
 add_dep+0x88/0x11c
 do_event.constprop.0+0x19c/0x2c0
 dept_event+0xe0/0x100
 complete+0x48/0x98
 mm_release+0x7c/0x90
 exit_mm_release+0xc/0x14
 do_exit+0x1b4/0x81c
 do_group_exit+0x30/0x9c
 __wake_up_parent+0x0/0x24
 invoke_syscall.constprop.0+0x78/0xc4
 do_el0_svc+0x98/0xd0
 el0_svc+0x44/0xe4
 el0t_64_sync_handler+0xb0/0x12c
 el0t_64_sync+0x158/0x15c


===================================================
DEPT: Circular dependency has been detected.
5.17.0-rc1+ #3 Tainted: G        W        
---------------------------------------------------
summary
---------------------------------------------------
*** AA DEADLOCK ***

context A
    [S] (unknown)(&try_completion:0)
    [W] wait_for_completion_timeout(&try_completion:0)
    [E] complete(&try_completion:0)

[S]: start of the event context
[W]: the wait blocked
[E]: the event not reachable
---------------------------------------------------
context A's detail
---------------------------------------------------
context A
    [S] (unknown)(&try_completion:0)
    [W] wait_for_completion_timeout(&try_completion:0)
    [E] complete(&try_completion:0)

[S] (unknown)(&try_completion:0):
(N/A)

[W] wait_for_completion_timeout(&try_completion:0):
[<ffffffc008166bf4>] kunit_try_catch_run+0xb4/0x160
stacktrace:
      dept_wait+0x74/0x88
      wait_for_completion_timeout+0x64/0xa0
      kunit_try_catch_run+0xb4/0x160
      kunit_test_try_catch_successful_try_no_catch+0x3c/0x98
      kunit_try_run_case+0x9c/0xa0
      kunit_generic_run_threadfn_adapter+0x1c/0x28
      kthread+0xd4/0xe4
      ret_from_fork+0x10/0x20

[E] complete(&try_completion:0):
[<ffffffc00803dce4>] kthread_complete_and_exit+0x18/0x20
stacktrace:
      dept_event+0xe0/0x100
      complete+0x48/0x98
      kthread_complete_and_exit+0x18/0x20
      kunit_try_catch_throw+0x0/0x1c
      kthread+0xd4/0xe4
      ret_from_fork+0x10/0x20

---------------------------------------------------
information that might be helpful
---------------------------------------------------
CPU: 15 PID: 132 Comm: kunit_try_catch Tainted: G        W         5.17.0-rc1+ #3
Hardware name: linux,dummy-virt (DT)
Call trace:
 dump_backtrace.part.0+0x9c/0xc4
 show_stack+0x14/0x28
 dump_stack_lvl+0x9c/0xcc
 dump_stack+0x14/0x2c
 print_circle+0x2d4/0x438
 cb_check_dl+0x44/0x70
 bfs+0x60/0x168
 add_dep+0x88/0x11c
 do_event.constprop.0+0x19c/0x2c0
 dept_event+0xe0/0x100
 complete+0x48/0x98
 kthread_complete_and_exit+0x18/0x20
 kunit_try_catch_throw+0x0/0x1c
 kthread+0xd4/0xe4
 ret_from_fork+0x10/0x20


> Benifit:
> 
> 	0. Works with all lock primitives.
> 	1. Works with wait_for_completion()/complete().
> 	2. Works with 'wait' on PG_locked.
> 	3. Works with 'wait' on PG_writeback.
> 	4. Works with swait/wakeup.
> 	5. Works with waitqueue.
> 	6. Multiple reports are allowed.
> 	7. Deduplication control on multiple reports.
> 	8. Withstand false positives thanks to 6.
> 	9. Easy to tag any wait/event.
> 
> Future work:

[...]

> -- 
> 1.9.1
> 

-- 
Thank you, You are awesome!
Hyeonggon :-)