Re: [PATCH v2] locking/lockdep: add debug_show_all_lock_holders()

[Waiman Long <longman@redhat.com>]

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>