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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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?