linux-kernel - Re: [PATCH] watchdog: print all locks on a softlock

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Message-ID: <20140501191720.GA198341@redhat.com>
Date:	Thu, 1 May 2014 15:17:20 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, Ben Zhang <benzh@...omium.org>
Subject: Re: [PATCH] watchdog: print all locks on a softlock

On Thu, May 01, 2014 at 02:55:35PM -0400, Eric Paris wrote:
> If the CPU hits a softlockup this patch will also have it print the
> information about all locks being held on the system.  This might help
> determine if a lock is being held too long leading to this problem.

I am not sure this helps you.  A softlockup is the result of pre-emption
disabled, ie the scheduler not being called after 60 seconds.  Holding a
lock does not disable pre-emption usually.  So I don't think this is going
to add anything.

Are you trying to debug a hung task?  The the hung_task thread checks to
see if a task hasn't scheduled in 2 minutes or so.  That could be the
result of long lock (but that output already dumps the lockdep stuff).

Cheers,
Don

> 
> Signed-off-by: Eric Paris <eparis@...hat.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Don Zickus <dzickus@...hat.com>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Ben Zhang <benzh@...omium.org>
> ---
>  kernel/watchdog.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 516203e..a027240 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -322,6 +322,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  			current->comm, task_pid_nr(current));
>  		print_modules();
>  		print_irqtrace_events(current);
> +		debug_show_all_locks();
>  		if (regs)
>  			show_regs(regs);
>  		else
> -- 
> 1.9.0
> 
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