Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Feb 17, 2009 at 08:28:10PM +0100, Oleg Nesterov wrote:
> On 02/17, Nick Piggin wrote:
> >
> > How's this?
> 
> To me, this patch makes the code much more clean/understandable.
> 
> And imho it is very good it removes smp_read_barrier_depends()s
> which (I think) were just wrong.
> 
> 
> But I still have the question,
> 
> > Does any architecture actually needs barriers? For the initiator I
> > could see it, but for the handler I would be surprised. The other
> > thing we could do for simplicity is just to require that a full
> > barrier is required before generating an IPI, and after receiving an
> > IPI. We can't just do that in generic code without auditing
> > architectures. There have been subtle hangs here on some archs in
> > the past.
> 
> OK, so we add the barrier here:
> 
> > @@ -104,6 +111,14 @@ void generic_smp_call_function_interrupt
> >  	int cpu = get_cpu();
> >
> >  	/*
> > +	 * Ensure entry is visible on call_function_queue after we have
> > +	 * entered the IPI. See comment in smp_call_function_many.
> > +	 * If we don't have this, then we may miss an entry on the list
> > +	 * and never get another IPI to process it.
> > +	 */
> > +	smp_mb();
> 
> But, any arch which needs this barrier should also call mb() in, say,
> smp_reschedule_interrupt() path. Otherwise we can miss TIF_NEED_RESCHED
> after return from the handler.
> 
> So the question is: is there any arch which surely needs this barrier?
> 
> IOW,
> 	int COND;
> 
> 	void smp_xxx_interrupt(regs)
> 	{
> 		BUG_ON(!COND);
> 	}
> 
> 	COND = 1;
> 	mb();
> 	smp_send_xxx(cpu);
> 
> can we really hit the BUG_ON() above on some arch?

If all of the above is executed by the same task, tripping the BUG_ON()
means either a compiler or CPU bug.

							Thanx, Paul

> (but in any case I agree, it is better to be safe and add the barrier
>  like this patch does).
> 
> Oleg.
>