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

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Date:	Tue, 17 Feb 2009 20:28:10 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())

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?


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

Oleg.

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