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

[Tetsuo Handa]

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@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Waiman Long <longman@xxxxxxxxxx>