linux-kernel - Re: [PATCH 1/1] rtmutex: Handle when top lock owner changes

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Date:	Wed, 4 Jun 2014 21:53:16 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Brad Mouring <bmouring@...com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 1/1] rtmutex: Handle when top lock owner changes

On Wed, 4 Jun 2014, Brad Mouring wrote:
> On Wed, Jun 04, 2014 at 08:02:16PM +0200, Thomas Gleixner wrote:
> > I'll fixup the check so it wont break the real deadlock case and queue
> > it.
> 
> How would the change break the real deadlock case?

> >  	/* Deadlock detection */
> >  	if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
> > +		/*
> > +		 * If the prio chain has changed out from under us, set the task
> > +		 * to the current owner of the lock in the current waiter and
> > +		 * continue walking the prio chain
> > +		 */
> > +		if (rt_mutex_owner(lock) && rt_mutex_owner(lock) != task) {

No, sorry. That's wrong.

Your change wreckages the rt_mutex_owner(lock) == top_task test
simply because in that case:

   (rt_mutex_owner(lock) && rt_mutex_owner(lock) != task)

evaluates to true.

So we want this:

Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -375,6 +375,26 @@ static int rt_mutex_adjust_prio_chain(st
 	 * walk, we detected a deadlock.
 	 */
 	if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
+		/*
+		 * If the prio chain has changed out from under us, set the task
+		 * to the current owner of the lock in the current waiter and
+		 * continue walking the prio chain
+		 */
+		if (rt_mutex_owner(lock) && rt_mutex_owner(lock) != task &&
+		    rt_mutex_owner(lock) != top_task) {
+			/* Release the old owner */
+			raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+			put_task_struct(task);
+
+			/* Move to the new owner */
+			task = rt_mutex_owner(lock);
+			get_task_struct(task);
+
+			/* Let's try this again */
+			raw_spin_unlock(&lock->wait_lock);
+			goto retry;
+		}
+
 		debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
 		raw_spin_unlock(&lock->wait_lock);
 		ret = deadlock_detect ? -EDEADLK : 0;
--
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