Re: [PATCH v3] locking/lockdep: add debug_show_all_lock_holders()

[Tetsuo Handa]

Ingo or Peter, are you there?

Linus is expecting that this patch (if acceptable) is sent from the locking people.

https://lkml.kernel.org/r/CAHk-=wjbu9USn=hVWQ9v9t1H+8R6qXj8REkm36==w10zM0cM6g@xxxxxxxxxxxxxx

On 2023/02/02 22:59, 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.
> 
> debug_show_all_lock_holders() skips current thread if the caller is
> holding no lock, for reporting RCU lock taken inside that function is
> generally useless.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> ---
> Changes in v3:
>   Unshare debug_show_all_lock_holders() and debug_show_all_locks(),
>   suggested by Ingo Molnar <mingo@xxxxxxxxxx>.
> 
> Changes in v2:
>   Share debug_show_all_lock_holders() and debug_show_all_locks(),
>   suggested by Waiman Long <longman@xxxxxxxxxx>.
> 
>  include/linux/debug_locks.h |  5 +++++
>  kernel/hung_task.c          |  2 +-
>  kernel/locking/lockdep.c    | 28 ++++++++++++++++++++++++++++
>  3 files changed, 34 insertions(+), 1 deletion(-)
>