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From: Vegard Nossum <vegard.nossum@gmail.com>
Date: Tue, 20 Dec 2016 09:14:40 +0100
Message-ID: <CAOMGZ=GyNBu+tGUQoQk6avLGU3L2FrvE7_8V+DetVZQJHKQ-jA@mail.gmail.com>
Subject: Re: [PATCH] locking/hung_task: Defer showing held locks
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Mandeep Singh Baines <msb@chromium.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>
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On 13 December 2016 at 15:45, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
> When I was running my testcase which may block hundreds of threads
> on fs locks, I got lockup due to output from debug_show_all_locks()
> added by commit b2d4c2edb2e4f89a ("locking/hung_task: Show all locks").
>
> I think we don't need to call debug_show_all_locks() on each blocked
> thread. Let's defer calling debug_show_all_locks() till before panic()
> or leaving for_each_process_thread() loop.

First of all, sorry for not answering earlier.

I'm not sure I fully understand the problem, you say the "output from
debug_show_all_locks()" caused a lockup, but was the problem simply
that the amount of output caused it to stall for a long time?

Could we instead

1) move the debug_show_all_locks() into the if
(sysctl_hung_task_panic) bit unconditionally

2) call something (touch_nmi_watchdog()?) inside debug_show_all_locks()

3) in another way make debug_show_all_locks() more robust so it doesn't "lockup"

?


Vegard