Date: Wed, 15 Apr 2015 12:26:58 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: mingo@kernel.org, linux-kernel@vger.kernel.org,
        Ben Segall <bsegall@google.com>, Roman Gushchin <klamm@yandex-team.ru>,
        Paul Turner <pjt@google.com>
Subject: Re: [PATCH 1/3] hrtimer: Fix race between hrtimer_start() and
 __run_hrtimer()
In-Reply-To: <20150415095011.743749536@infradead.org>
Message-ID: <alpine.DEB.2.11.1504151209510.3845@nanos>
References: <20150415094155.601987867@infradead.org> <20150415095011.743749536@infradead.org>
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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.

So I rather prefer a subject line like this

hrtimer: Allow concurrent hrtimer_start() for self restarting timers

or such.

> Because we drop cpu_base->lock around calling hrtimer::function, it is
> possible for hrtimer_start() to come in between and enqueue the timer.
> 
> If hrtimer::function then returns HRTIMER_RESTART we'll hit the BUG_ON
> because HRTIMER_STATE_ENQUEUED will be set.
> 
> Since the above is a perfectly valid scenario, remove the BUG_ON and
> make the enqueue_hrtimer() call conditional on the timer not being
> enqueued already.
> 
> NOTE: in that concurrent scenario its entirely common for both sites
> to want to modify the hrtimer, since hrtimers don't provide
> serialization themselves be sure to provide some such that the
> hrtimer::function and the hrtimer_start() caller don't both try and
> fudge the expiration state at the same time.

Right.
 
> 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()?

Other than that I can see why you want that ...

Thanks,

	tglx
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