Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer

[Kirill Tkhai]

Ð ÐÑ, 10/06/2015 Ð 18:04 +0200, Oleg Nesterov ÐÐÑÐÑ:
> Hi Kirill,
> 
> On 06/10, Kirill Tkhai wrote:
> >
> > Ð ÐÑ, 09/06/2015 Ð 23:33 +0200, Oleg Nesterov ÐÐÑÐÑ:
> > >
> > > 	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?

I'm agree with you. Thanks for the explanation.

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