Re: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invokedwith irqs disabled

From: Paul E. McKenney
Date: Sat Jul 23 2011 - 18:04:03 EST

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 ( 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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