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

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Message-ID: <5f853677-874a-2a38-4253-95b2795eb7a8@redhat.com>
Date:   Tue, 4 Oct 2022 14:09:39 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        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()

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>