linux-kernel - Re: [PATCH v2] locking/lockdep: add debug_show_all_lock

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Date:   Mon, 14 Nov 2022 19:05:51 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Dooks <ben.dooks@...ive.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Xiaoming Ni <nixiaoming@...wei.com>,
        John Ogness <john.ogness@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] locking/lockdep: add debug_show_all_lock_holders()

Peter or Ingo, can you take this patch?

On 2022/10/05 3:09, Waiman Long wrote:
> On 10/3/22 18:18, Tetsuo Handa wrote:
>> Can this patch go to linux.git ?
> 
> It is now up to Peter or Ingo to take it  to tip.
> 
> Cheers,
> Longman
> 
>>
>> On 2022/09/17 3:41, Waiman Long wrote:
>>> On 9/16/22 11:57, Tetsuo Handa wrote:
>>>> Currently, check_hung_uninterruptible_tasks() reports details of locks
>>>> held in the system. Also, lockdep_print_held_locks() does not report
>>>> details of locks held by a thread if that thread is in TASK_RUNNING state.
>>>> Several years of experience of debugging without vmcore tells me that
>>>> these limitations have been a barrier for understanding what went wrong
>>>> in syzbot's "INFO: task hung in" reports.
>>>>
>>>> I initially thought that the cause of "INFO: task hung in" reports is
>>>> due to over-stressing. But I understood that over-stressing is unlikely.
>>>> I now consider that there likely is a deadlock/livelock bug where lockdep
>>>> cannot report as a deadlock when "INFO: task hung in" is reported.
>>>>
>>>> A typical case is that thread-1 is waiting for something to happen (e.g.
>>>> wait_event_*()) with a lock held. When thread-2 tries to hold that lock
>>>> using e.g. mutex_lock(), check_hung_uninterruptible_tasks() reports that
>>>> thread-2 is hung and thread-1 is holding a lock which thread-2 is trying
>>>> to hold. But currently check_hung_uninterruptible_tasks() cannot report
>>>> the exact location of thread-1 which gives us an important hint for
>>>> understanding why thread-1 is holding that lock for so long period.
>>>>
>>>> When check_hung_uninterruptible_tasks() reports a thread waiting for a
>>>> lock, it is important to report backtrace of threads which already held
>>>> that lock. Therefore, allow check_hung_uninterruptible_tasks() to report
>>>> the exact location of threads which is holding any lock.
>>>>
>>>> To deduplicate code, share debug_show_all_{locks,lock_holders}() using
>>>> a flag. As a side effect of sharing, __debug_show_all_locks() skips
>>>> current thread if the caller is holding no lock, for reporting RCU lock
>>>> taken inside __debug_show_all_locks() is generally useless.
>>>>
>>>> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>>> Acked-by: Waiman Long <longman@...hat.com>
>