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

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Date:	Wed, 4 Jun 2014 14:25:41 -0500
From:	"Brad Mouring" <bmouring@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Brad Mouring <bmouring@...com>,
	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, Jun 04, 2014 at 08:02:16PM +0200, Thomas Gleixner wrote:
> On Wed, 4 Jun 2014, Steven Rostedt wrote:
> 
> > On Wed, 4 Jun 2014 17:32:37 +0200 (CEST)
> > Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> > > T3 releases L3
> > > T2 gets L3
> > > T2 drops L3 and L2
> > > T2 blocks on L4 held by T4
> > > T4 blocked on L5 held by T5
> > > 
> > > So we happily boost T4 and T5. Not what we really want to do.
> > > 
> > > Nasty, isn't it ?
> > > 
> > 
> > Actually, we may go up a chain, but we never do any unnecessary
> > boosting. That's because the boost is done with rt_mutex_adjust_prio()
> > which gets the prio from rt_mutex_getprio() which reads the
> > task->normal_prio and compares it to the task_top_pi_waiter(task)->prio,
> > which will always be correct as we have the necessary locks.
> 
> Indeed.

I had thought through how to try to determine, from what we knew at
this point, whether or not we were walking a different chain for just
this concern, but I had convinced myself that it would be cumbersome,
error-prone, and, as pointed out here, not vital.

>  
> > And we don't even need to worry about the chain we miss. That is, if
> > task A is blocked on a lock owned by D at the time, but as we go up the
> > chain, D releases the lock and B grabs it, B will still up its priority
> > based on the waiters of the lock (that is A), and if B blocks, it will
> > boost the tasks that own the lock it blocks on, where B is still
> > influenced by A.
> > 
> > The fact that we only update the prio based on the actual waiters and
> > don't carry a prio up the chain (which you designed, and I thought was
> > quite ingenious by the way), we may waste time going up a chain, but
> > the priority inheritance is still accurate.
> 
> Duh. I actually had to lookup my notes from back then. There is even a
> lenghty IRC discussion about not propagating the least waiters prio,
> but lookup the actual lock waiters. Good, so we just walk for nothing
> and waste some cpu cycles.
> 
I put the check within the deadlock detection block to try to minimize
this case. As evidenced by the spinning on this, it's a rare case where
your userspace code has to be doing some wacky (but valid) stuff.

> My brain still suffers from 3 days staring into futex.c
> 
> 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?

> 
> Thanks,
> 
> 	tglx
> 
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