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Message-ID: <5f853677-874a-2a38-4253-95b2795eb7a8@redhat.com>
Date:   Tue, 4 Oct 2022 14:09:39 -0400
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Subject: Re: [PATCH v2] locking/lockdep: add debug_show_all_lock_holders()
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To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Shaokun Zhang <zhangshaokun@hisilicon.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Petr Mladek <pmladek@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ben Dooks <ben.dooks@sifive.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        John Ogness <john.ogness@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
References: <d5393b0e-a296-3296-d376-c9178669747b@I-love.SAKURA.ne.jp>
 <3e027453-fda4-3891-3ec3-5623f1525e56@redhat.com>
 <9f42e8a5-f809-3f2c-0fda-b7657bc94eb3@I-love.SAKURA.ne.jp>
 <1fd381e0-03e7-df2a-4865-d157714ce9b2@redhat.com>
 <1f71cfbf-7bee-6a06-dca1-ac94bf542cd7@I-love.SAKURA.ne.jp>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <1f71cfbf-7bee-6a06-dca1-ac94bf542cd7@I-love.SAKURA.ne.jp>
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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@I-love.SAKURA.ne.jp>
>> Acked-by: Waiman Long <longman@redhat.com>