linux-kernel - Re: [PATCH 1/3] hrtimer: Fix race between hrtimer_start() and __run

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Date:	Wed, 15 Apr 2015 13:35:15 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	Ben Segall <bsegall@...gle.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 1/3] hrtimer: Fix race between hrtimer_start() and
 __run_hrtimer()

On Wed, 15 Apr 2015, Peter Zijlstra wrote:
> On Wed, Apr 15, 2015 at 12:26:58PM +0200, Thomas Gleixner wrote:
> > On Wed, 15 Apr 2015, Peter Zijlstra wrote:
> > > hrtimer: Fix race between hrtimer_start() and __run_hrtimer()
> > 
> > I don't think that subject line is correct.
> > 
> > Back in the early hrtimer days we made deliberately the design
> > decision that this kind of usage is forbidden. The reason for this is
> > that the hrtimer infrastructure cannot provide proper
> > serialization. So we thought it would be a sane restruction that
> > restarting a timer from the callback should not be mixed with
> > concurrent restarts from a different call site.
> 
> Ah I was not aware. Until I changed the locking it was possible simply
> because everything was serialized by the base lock. So the concurrent
> start would either land before the callback or after it but not in the
> middle like it can now.
> 
> > So I rather prefer a subject line like this
> > 
> > hrtimer: Allow concurrent hrtimer_start() for self restarting timers
> > 
> 
> /me copy/paste, done! :-)
> 
> > > To that effect, add a WARN when someone tries to forward an already
> > > enqueued timer.
> > 
> > The warnon itself is nice, but what about sites which use
> > hrtimer_set_expires() and hrtimer_start_expires()?
> 
> They are all inlines, furthermore forward is the most common way to
> change the expiry of periodic / self restarting timers so would gain us
> most.

Right. I just wanted to mention it.

> How about this then?

Looks good. Should I add those 3 patches to the other pile of hrtimer
stuff?
 
Thanks,

	tglx
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