linux-kernel - Re: workqueue code needing preemption disabled

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Message-ID: <1363624243.25967.178.camel@gandalf.local.home>
Date:	Mon, 18 Mar 2013 12:30:43 -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 12:27 -0400, Steven Rostedt wrote:

> 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.

If you happen to know the critical areas that require preemption to be
disabled for real, we can encapsulate them with:

	preempt_disable_rt();

	preempt_enable_rt();

These are currently only in the -rt patch, but it annotates locations
that require preemption to be disabled even when -rt converts spin_locks
into mutexes. These obviously can not contain spin_locks() as
spin_locks() can block and schedule out.

-- Steve


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