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Date: Tue, 17 Feb 2009 22:45:18 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Nick Piggin <npiggin@suse.de>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Jens Axboe <jens.axboe@oracle.com>,
       Suresh Siddha <suresh.b.siddha@intel.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>, Rusty Russell <rusty@rustcorp.com.au>,
       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       linux-arch@vger.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())
Message-ID: <20090217214518.GA13189@redhat.com>
References: <1234820704.30178.396.camel@laptop> <20090216220214.GA10093@redhat.com> <1234823097.30178.406.camel@laptop> <20090216231946.GA12009@redhat.com> <1234862974.4744.31.camel@laptop> <20090217101130.GA8660@wotan.suse.de> <1234866453.4744.58.camel@laptop> <20090217112657.GE26402@wotan.suse.de> <20090217192810.GA4980@redhat.com> <20090217213256.GJ6761@linux.vnet.ibm.com>
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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.