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 22:45:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	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>,
	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, Paul E. McKenney wrote:
>
> On Tue, Feb 17, 2009 at 08:28:10PM +0100, Oleg Nesterov wrote:
> >
> > 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.

I think you misunderstood...

smp_send_xxx() sends the ipi to another CPU, and smp_xxx_interrupt() is
the handler.

Oleg.

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