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Message-ID: <5f853677-874a-2a38-4253-95b2795eb7a8@redhat.com> Date: Tue, 4 Oct 2022 14:09:39 -0400 From: Waiman Long <longman@...hat.com> To: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com> Cc: Thomas Gleixner <tglx@...utronix.de>, Shaokun Zhang <zhangshaokun@...ilicon.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Petr Mladek <pmladek@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, Ben Dooks <ben.dooks@...ive.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Luis Chamberlain <mcgrof@...nel.org>, Xiaoming Ni <nixiaoming@...wei.com>, John Ogness <john.ogness@...utronix.de>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v2] locking/lockdep: add debug_show_all_lock_holders() 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@...ove.SAKURA.ne.jp> >> Acked-by: Waiman Long <longman@...hat.com>
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