linux-kernel - Re: workqueue code needing preemption disabled

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Message-ID: <20130318162718.GB20133@mtj.dyndns.org>
Date:	Mon, 18 Mar 2013 09:27:18 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.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

Hey, Steven.

On Mon, Mar 18, 2013 at 12:23:19PM -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)". Because, in -rt, interrupts
> (except for the timer interrupt) are run as threads, and anything that
> isn't marked as raw_spin_lock() turns into a mutex. I don't believe it's
> safe to turn the gcwq->lock into a raw_spin_lock either, or at least not
> short enough to hold it. Anything that holds a spin_lock() for more than
> a microsecond is too much for a raw lock.

Does that mean that a task holding gcwq->lock may be preempted?  If
so, that sure could lead to weird problems.  Maybe gcwq->lock should
be marked non-preemptible somehow?

Thanks.

-- 
tejun
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