linux-kernel - Re: Filesystem lockup with CONFIG_PREEMPT

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Date:	Thu, 26 Jun 2014 12:50:24 -0700
From:	Austin Schuh <austin@...oton-tech.com>
To:	Richard Weinberger <richard.weinberger@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: Filesystem lockup with CONFIG_PREEMPT_RT

On Wed, May 21, 2014 at 12:33 AM, Richard Weinberger
<richard.weinberger@...il.com> wrote:
> CC'ing RT folks
>
> On Wed, May 21, 2014 at 8:23 AM, Austin Schuh <austin@...oton-tech.com> wrote:
>> On Tue, May 13, 2014 at 7:29 PM, Austin Schuh <austin@...oton-tech.com> wrote:
>>> Hi,
>>>
>>> I am observing a filesystem lockup with XFS on a CONFIG_PREEMPT_RT
>>> patched kernel.  I have currently only triggered it using dpkg.  Dave
>>> Chinner on the XFS mailing list suggested that it was a rt-kernel
>>> workqueue issue as opposed to a XFS problem after looking at the
>>> kernel messages.

I've got a 100% reproducible test case that doesn't involve a
filesystem.  I wrote a module that triggers the bug when the device is
written to, making it easy to enable tracing during the event and
capture everything.

It looks like rw_semaphores don't trigger wq_worker_sleeping to run
when work goes to sleep on a rw_semaphore.  This only happens with the
RT patches, not with the mainline kernel.  I'm foreseeing a second
deadlock/bug coming into play shortly.  If a task holding the work
pool spinlock gets preempted, and we need to schedule more work from
another worker thread which was just blocked by a mutex, we'll then
end up trying to go to sleep on 2 locks at once.

That is getting a bit deep into the scheduler for me...  Any
suggestions on how to fix it?

Austin

View attachment "killer_module.c" of type "text/x-csrc" (4183 bytes)