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Date:   Wed, 2 Mar 2022 04:36:38 +0000
From:   Hyeonggon Yoo <>
To:     Byungchul Park <>
Subject: Re: [PATCH v3 00/21] DEPT(Dependency Tracker)

On Mon, Feb 28, 2022 at 06:56:39PM +0900, Byungchul Park wrote:
> I didn't want to bother you so I was planning to send the next spin
> after making more progress. However, PATCH v2 reports too many false
> positives because Dept tracked the bit_wait_table[] wrong way - I
> apologize for that. So I decided to send PATCH v3 first before going
> further for those who want to run Dept for now.
> There might still be some false positives but not overwhelming.

Hello Byungchul, I'm running DEPT v3 on my system
and I see report below.

Looking at the kmemleak code and comment, I think
kmemleak tried to avoid lockdep recursive warning
but detected by DEPT?

DEPT: Circular dependency has been detected.
5.17.0-rc1+ #1 Tainted: G        W

context A
    [S] __raw_spin_lock_irqsave(&object->lock:0)
    [W] _raw_spin_lock_nested(&object->lock:0)
    [E] spin_unlock(&object->lock:0)

[S]: start of the event context
[W]: the wait blocked
[E]: the event not reachable
context A's detail
context A
    [S] __raw_spin_lock_irqsave(&object->lock:0)
    [W] _raw_spin_lock_nested(&object->lock:0)
    [E] spin_unlock(&object->lock:0)

[S] __raw_spin_lock_irqsave(&object->lock:0):
[<ffffffc00810302c>] scan_gray_list+0x84/0x13c

[E] spin_unlock(&object->lock:0):
[<ffffffc008102ee0>] scan_block+0x60/0x128
information that might be helpful
CPU: 1 PID: 38 Comm: kmemleak Tainted: G        W         5.17.0-rc1+ #1
Hardware name: linux,dummy-virt (DT)
Call trace:

> ---
> 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.
> 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:
> 	0. To make it more stable.
> 	1. To separates Dept from Lockdep.
> 	2. To improves performance in terms of time and space.
> 	3. To use Dept as a dependency engine for Lockdep.
> 	4. To add any missing tags of wait/event in the kernel.
> 	5. To deduplicate stack trace.
> How to interpret reports:
> 	1. E(event) in each context cannot be triggered because of the
> 	   W(wait) that cannot be woken.
> 	2. The stack trace helping find the problematic code is located
> 	   in each conext's detail.
> Thanks,
> Byungchul
> ---
> Changes from v2:
> 	1. Disable Dept on bit_wait_table[] in sched/wait_bit.c
> 	   reporting a lot of false positives, which is my fault.
> 	   Wait/event for bit_wait_table[] should've been tagged in a
> 	   higher layer for better work, which is a future work.
> 	   (feedback from Jan Kara)
> 	2. Disable Dept on crypto_larval's completion to prevent a false
> 	   positive.
> Changes from v1:
> 	1. Fix coding style and typo. (feedback from Steven)
> 	2. Distinguish each work context from another in workqueue.
> 	3. Skip checking lock acquisition with nest_lock, which is about
> 	   correct lock usage that should be checked by Lockdep.
> Changes from RFC:
> 	1. Prevent adding a wait tag at prepare_to_wait() but __schedule().
> 	   (feedback from Linus and Matthew)
> 	2. Use try version at lockdep_acquire_cpus_lock() annotation.
> 	3. Distinguish each syscall context from another.
[ ... ] 

Thank you, You are awesome!
Hyeonggon :-)

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