linux-kernel - Re: [RFC][PATCH 8/8] rtmutex: Fix PI chain order integrity

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Date:	Tue, 28 Jun 2016 10:05:53 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, tglx@...utronix.de, rostedt@...dmis.org,
	xlpang@...hat.com, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...icios.com, jdesfossez@...icios.com,
	bristot@...hat.com
Subject: Re: [RFC][PATCH 8/8] rtmutex: Fix PI chain order integrity

On 27/06/16 14:23, Peter Zijlstra wrote:
> On Wed, Jun 15, 2016 at 08:25:07AM +0100, Juri Lelli wrote:
> > I guess it's not that likely, but yes it could potentially happen that a
> > waiter is optimistically spinning, depletes its runtime, gets throttled
> > and then replenished when still spinning. Maybe it doesn't really make
> > sense continuing spinning in this situation, but I guess things get
> > really complicated. :-/
> > 
> > Anyway, as said, I think this patch is OK. Maybe we want to add a
> > comment just to remember what situation can cause an issue if we don't
> > do this? Patch changelog would be OK as well for such a comment IMHO.
> 
> 
> OK, so I went to write a simple comment and ended up with the below :/
> 
> While writing the comment I noticed two issues:
> 
>  - we update the waiter order fields while the entry is still enqueued
>    on the pi_waiters tree, which is also sorted by these exact fields.
> 
>  - another one of these pure ->prio comparisons
> 
> Please double check, there be dragons here.
> 

Reviewed-and-tested-by: Juri Lelli <juri.lelli@....com>

And, FWIW, you can obviously keep this if you are going to squash this
into 8/8.

Best,

- Juri