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Date:   Tue, 4 Oct 2022 07:18:23 +0900
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Subject: Re: [PATCH v2] locking/lockdep: add debug_show_all_lock_holders()
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To:     Waiman Long <longman@redhat.com>,
        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>
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
In-Reply-To: <1fd381e0-03e7-df2a-4865-d157714ce9b2@redhat.com>
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Can this patch go to linux.git ?

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>