linux-kernel - Re: workqueue code needing preemption disabled

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Message-ID: <1363624020.25967.175.camel@gandalf.local.home>
Date:	Mon, 18 Mar 2013 12:27:00 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Clark Williams <clark@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: workqueue code needing preemption disabled

On Mon, 2013-03-18 at 12:23 -0400, Steven Rostedt wrote:

> > Maybe I'm confused but I can't really see how the above would be a
> > problem to workqueue in itself.  Both rq->lock and gcwq->lock are
> > irq-safe, so spin_lock() not disabling preemption shouldn't be a
> > problem.  Are CPU hotplug operations involved?
> 
> No CPU hotplug is involved here. But I will note that gcwq->lock in -rt
> is not irq -safe. That is, in rt the spin_lock_irq(&gcwq->lock) really
> becomes a special "mutex_lock(&gcwq->lock)".

IOW, what can happen in -rt here is:

	spin_lock_irq(&gcwq->lock);
	[...]
	<interrupt>
		-> preempt_schedule();
		   schedule();
		   try_to_wake_up_local();

	[...]
	spin_unlock_irq(&gcwq->lock);

Again, with -rt, spin_lock_irq() does not prevent interrupts nor
preemption.

-- Steve


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