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Date: Thu, 30 Oct 2008 18:25:15 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: "paulmck@linux.vnet.ibm.com" <paulmck@linux.vnet.ibm.com>,
       "mingo@elte.hu" <mingo@elte.hu>,
       "jeremy.fitzhardinge@citrix.com" <jeremy.fitzhardinge@citrix.com>,
       "nickpiggin@yahoo.com.au" <nickpiggin@yahoo.com.au>,
       "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "Mallick, Asit K" <asit.k.mallick@intel.com>
Subject: Re: [patch] generic-ipi: fix the smp_mb() placement
Message-ID: <20081030172515.GW31673@kernel.dk>
References: <20081029224229.GK30573@linux-os.sc.intel.com> <20081030072029.GK31673@kernel.dk> <20081030163008.GL30573@linux-os.sc.intel.com>
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On Thu, Oct 30 2008, Suresh Siddha wrote:
> On Thu, Oct 30, 2008 at 12:20:30AM -0700, Jens Axboe wrote:
> > > @@ -76,6 +72,11 @@ static void generic_exec_single(int cpu, struct call_single_data *data)
> > >  	list_add_tail(&data->list, &dst->list);
> > >  	spin_unlock_irqrestore(&dst->lock, flags);
> > >  
> > > +	/*
> > > +	 * Make the list addition visible before sending the ipi.
> > > +	 */
> > > +	smp_mb();
> > > +
> > >  	if (ipi)
> > >  		arch_send_call_function_single_ipi(cpu);
> > 
> > We can downgrade this to a smp_wmb().
> 
> No. We want the ipi receiver to see the new consistent data rather
> than possible old consistent data.

Oh right, we need visibility here and not just store ordering.

> 
> And on x86, smp_wmb() is a simple barrier() (in !CONFIG_X86_OOSTORE)
> and which doesn't do much in this case.
> 
> on x86 mfence (smp_mb()) will ensure that msr based APIC (x2apic)
> accesses (ipi) will be visible only after the memory operations before
> smp_mb() are made visible.

OK, I'm convinced. I'll queue up the patch, thanks!

-- 
Jens Axboe