linux-kernel - Re: [PATCH 0/3] hrtimer: HRTIMER_STATE

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Date:	Mon, 8 Jun 2015 22:52:46 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>, umgwanakikbuti@...il.com,
	mingo@...e.hu, ktkhai@...allels.com, rostedt@...dmis.org,
	juri.lelli@...il.com, pang.xunlei@...aro.org,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] hrtimer: HRTIMER_STATE_ fixes

On 06/08, Thomas Gleixner wrote:
>
> On Mon, 8 Jun 2015, Peter Zijlstra wrote:
> > On Mon, 2015-06-08 at 17:10 +0200, Oleg Nesterov wrote:
> > > On 06/08, Peter Zijlstra wrote:
> > > >
> > > > > I tend to agree, but I think its a pre-existing problem, not one
> > > > > introduced by my proposed patch.
> > > >
> > > > Something like this would fix that I think. It fully preserves
> > > > timer->state over hrtimer_start_range_ns().
> > >
> > > Yes, but I think we can do a bit better.
> > >
> > > Only for initial review, I need to re-check this...
> >
> > I'm having a wee bit of bother spotting how you version is 'better'.
> >
> > > And. I think that after you remove STATE_CALLBACK we can even kill
> > > timer->state altogether, but this is another story.
> >
> > Ah, yes, we could introduce timerqueue_is_queued() which uses
> > RB_EMPTY_NODE(). Obviating the need for hrtimer::state entirely.
>
> Which won't work for the migration case unless we have some trickery
> like we do with double linked lists (not setting the prev member to
> NULL on dequeue).

Of course, but this is trivial, no? Nevermind, I could easily miss
somthing and right now this is off-topic.

What do you think about this series? To me it makes sense in any case.
But I need (at least)to update the changelogs. In particular 3/3 doesn't
explain why do we need this change. If you missed the previous discussion,
this (hopefully) fixes the problem with the auto-rearming timers, the
"random" hrtimer_restart() wrongly creates a window when this timer looks
as !hrtimer_inactive().




Peter, I tried to think again about ->running and all I can say is that
I am totally confused ;) I'll try to write another email tomorrow.

Oleg.

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