linux-kernel - Re: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invoked with irqs disabled

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Message-ID: <20110723220348.GD2415@linux.vnet.ibm.com>
Date:	Sat, 23 Jul 2011 15:03:48 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	tglx@...utronix.de, laijs@...fujitsu.com, darren@...art.com,
	rostedt@...dmis.org, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invoked
 with irqs disabled

On Thu, Jul 21, 2011 at 01:32:48PM +0200, Peter Zijlstra wrote:
> On Tue, 2011-07-19 at 13:14 -0700, Paul E. McKenney wrote:
> > Because rcu_read_unlock() can be invoked with interrupts disabled, it can
> > in turn invoke rt_mutex_unlock() with interrupts disabled.  This situation
> > results in lockdep splats (https://lkml.org/lkml/2011/7/7/362) because the
> > rt_mutex structure's ->lock_wait is acquired elsewhere without disabling
> > interrupts, which can result in deadlocks.
> > 
> > This commit therefore changes the rt_mutex structure's ->lock_wait
> > acquisitions to disable interrupts.
> > 
> > An alternative fix is to prohibit invoking rcu_read_unlock() with
> > interrupts disabled unless the entire preceding RCU read-side critical
> > section has run with interrupts disabled.  However, there is already
> > at least one case in mainline where this potential rule is violated,
> > and there might well be many more.  These would likely be found one at
> > a time using the lockdep-water-torture method, hence the alternative
> > fix in the form of this commit.
> 
> Thomas, I'm inclined to merge this, any objections?

FWIW, it has been passing tests here.

							Thanx, Paul
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