linux-kernel - Re: Filesystem lockup with CONFIG_PREEMPT

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Date:	Fri, 27 Jun 2014 19:34:53 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Austin Schuh <austin@...oton-tech.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Weinberger <richard.weinberger@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: Filesystem lockup with CONFIG_PREEMPT_RT

On Fri, 2014-06-27 at 10:01 -0400, Steven Rostedt wrote:

> This seems like a lot of hacks.

It is exactly that, lacking proper pooper-scooper, show rt kernel how to
not step in it.

> I'm wondering if it would work if we
> just have the rt_spin_lock_slowlock not call schedule(), but call
> __schedule() directly. I mean it would keep with the mainline paradigm
> as spinlocks don't sleep there, and one going to sleep in the -rt
> kernel is similar to it being preempted by a very long NMI.

Problem being that we do sleep there, do need wakeup.  I have a hack
that turns them back into spinning locks, but it.. works too :)

> Does a spin_lock going to sleep really need to do all the presched and
> postsched work?

It would be lovely if we didn't have to do any of that.  On the IO bit,
I haven't seen hard evidence that the spinlock bit is absolutely
required (better not be, it doesn't guarantee anything), but the
combined hack did kill IO deadlock of multiple filesystems.

-Mike

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