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

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Date:	Thu, 1 May 2014 22:43:57 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Eric Paris <eparis@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	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

2014-05-01 22:09 GMT+02:00 Frederic Weisbecker <fweisbec@...il.com>:
> On Thu, May 01, 2014 at 03:17:20PM -0400, Don Zickus wrote:
>> 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).
>
> There may be some deadlocks that lockdep doesn't detect yet. 2 example:
>
> 1) spinlock <-> IPI dependency
>
>
>         CPU 0                                            CPU 1
>         --------------------------------------------------------
>         spin_lock_irq(A)
>         smp_send_function_single_async(CPU 1, func)
>                                                          //IPI
>                                                          func {
>                                                             spin_lock(1)
>                                                          }
>
> But this should be resolved with a virtual lock on the IPI functions.
> I should try that.

So actually this one above shouldn't be a problem because the _async
version doesn't wait for the IPI to complete. But the below still
looks possible.

>
> 2) rwlock <-> IPI
>
>         CPU 0                                            CPU 1
>         --------------------------------------------------------
>         read_lock(A)
>                                                          write_lock_irq(A)
>         smp_send_function_single(CPU 1, func)
>                                                          //IPI never happens
>
> This one is much trickier.
>
> Anyway those are the only scenario I know of but there may be more. When possible
> we want to extend lockdep to detect new scenarios of deadlock but we don't have the
> guarantee that it can detect everything.
>
> So, could be useful...
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