Date: Wed, 10 Jun 2015 18:04:44 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Kirill Tkhai <ktkhai@odin.com>
Cc: Peter Zijlstra <peterz@infradead.org>, umgwanakikbuti@gmail.com,
        mingo@elte.hu, ktkhai@parallels.com, rostedt@goodmis.org,
        tglx@linutronix.de, juri.lelli@gmail.com, pang.xunlei@linaro.org,
        wanpeng.li@linux.intel.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the
	timer
Message-ID: <20150610160444.GB3341@redhat.com>
References: <20150605084836.364306429@infradead.org> <20150605085205.723058588@infradead.org> <20150607223317.GA5193@redhat.com> <20150608091417.GM19282@twins.programming.kicks-ass.net> <20150608124234.GW18673@twins.programming.kicks-ass.net> <20150609213318.GA12436@redhat.com> <1433922411.23588.132.camel@odin.com>
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Hi Kirill,

On 06/10, Kirill Tkhai wrote:
>
> В Вт, 09/06/2015 в 23:33 +0200, Oleg Nesterov пишет:
> >
> > 	hrtimer_active(timer)
> > 	{
> >
> > 		do {
> > 			base = READ_ONCE(timer->base->cpu_base);
> > 			seq = read_seqcount_begin(&cpu_base->seq);
> >
> > 			if (timer->state & ENQUEUED ||
> > 			    base->running == timer)
> > 				return true;
> >
> > 		} while (read_seqcount_retry(&cpu_base->seq, seq) ||
> > 			 base != READ_ONCE(timer->base->cpu_base));
> >
> > 		return false;
> > 	}
> >
> > And we need to avoid the races with 2 transitions in __run_hrtimer().
> >
> > The first race is trivial, we change __run_hrtimer() to do
> >
> > 	write_seqcount_begin(cpu_base->seq);
> > 	cpu_base->running = timer;
> > 	__remove_hrtimer(timer);	// clears ENQUEUED
> > 	write_seqcount_end(cpu_base->seq);
>
> We use seqcount, because we are afraid that hrtimer_active() may miss
> timer->state or cpu_base->running, when we are clearing it.

Yes,

> If we use two pairs of write_seqcount_{begin,end} in __run_hrtimer(),
> we may protect only the places where we do that:
>
> 	cpu_base->running = timer;
> 	write_seqcount_begin(cpu_base->seq);
> 	__remove_hrtimer(timer);	// clears ENQUEUED
> 	write_seqcount_end(cpu_base->seq);
>
> 	....
>
> 	timer->state |= HRTIMER_STATE_ENQUEUED;
> 	write_seqcount_begin(cpu_base->seq);
> 	base->running = NULL;
> 	write_seqcount_end(cpu_base->seq);

Afaics, no. Afaics, the following code is correct:

	seqcount_t LOCK;
	bool X = true, Y = false;

	void read(void)
	{
		bool x, y;

		do {
			seq = read_seqcount_begin(&LOCK);

			x = X; y = Y;

		} while (read_seqcount_retry(&LOCK, seq));

		BUG_ON(!x && !y);
	}

	void write(void)
	{
		Y = true;

		write_seqcount_begin(LOCK);
		write_seqcount_end(LOCK);

		X = false;
	}

If we rely on the "locking" semantics of seqcount_t, this doesn't really
differ from

	spinlock_t LOCK;
	bool X = true, Y = false;

	void read(void)
	{
		bool x, y;

		spin_lock(LOCK);
		x = X; y = Y;
		spin_unlock(LOCK);

		BUG_ON(!x && !y);
	}

	void write(void)
	{
		Y = true;

		spin_lock(LOCK);
		spin_unlock(LOCK);

		X = false;
	}

If "read" takes the lock before "write", it must see X == true.

Otherwise "read" should see all memory changes done before or
inside the "write" critical section, so it must see Y == true.

No?

Oleg.

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