linux-kernel - Re: [RFC PATCH 09/10] POWERPC: smp: remove call to ipi_call_lock()/ipi_call

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Date:	Mon, 18 Jun 2012 10:51:59 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, ralf@...ux-mips.org,
	sshtylyov@...sta.com, david.daney@...ium.com,
	nikunj@...ux.vnet.ibm.com, axboe@...nel.dk, mingo@...nel.org,
	tglx@...utronix.de, peterz@...radead.org,
	akpm@...ux-foundation.org, srivatsa.bhat@...ux.vnet.ibm.com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 09/10] POWERPC: smp: remove call to
 ipi_call_lock()/ipi_call_unlock()

On Sat, Jun 16, 2012 at 09:32:19AM -0700, Paul E. McKenney wrote:
> On Tue, May 29, 2012 at 03:16:04PM +0800, Yong Zhang wrote:
> > From: Yong Zhang <yong.zhang@...driver.com>
> > 
> > 1) call_function.lock used in smp_call_function_many() is just to protect
> >    call_function.queue and &data->refs, cpu_online_mask is outside of the
> >    lock. And it's not necessary to protect cpu_online_mask,
> >    because data->cpumask is pre-calculate and even if a cpu is brougt up
> >    when calling arch_send_call_function_ipi_mask(), it's harmless because
> >    validation test in generic_smp_call_function_interrupt() will take care
> >    of it.
> > 
> > 2) For cpu down issue, stop_machine() will guarantee that no concurrent
> >    smp_call_fuction() is processing.
> 
> However, there is an effort to get rid of stop_machine() from the
> CPU-down path...  So something else will be needed.

Yeah. So Thomas changed the commit log like below:
[
    ipi_call_lock/unlock() lock resp. unlock call_function.lock. This lock
    protects only the call_function data structure itself, but it's
    completely unrelated to cpu_online_mask. The mask to which the IPIs
    are sent is calculated before call_function.lock is taken in
    smp_call_function_many(), so the locking around set_cpu_online() is
    pointless and can be removed.
    
    [ tglx: Massaged changelog ]
]

in tip/smp/hotplug.

Thanks,
Yong

> 
> 							Thanx, Paul
> 
> > Signed-off-by: Yong Zhang <yong.zhang0@...il.com>
> > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Cc: linuxppc-dev@...ts.ozlabs.org
> > ---
> >  arch/powerpc/kernel/smp.c |    2 --
> >  1 files changed, 0 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index e4cb343..e1417c4 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -571,7 +571,6 @@ void __devinit start_secondary(void *unused)
> >  	if (system_state == SYSTEM_RUNNING)
> >  		vdso_data->processorCount++;
> >  #endif
> > -	ipi_call_lock();
> >  	notify_cpu_starting(cpu);
> >  	set_cpu_online(cpu, true);
> >  	/* Update sibling maps */
> > @@ -601,7 +600,6 @@ void __devinit start_secondary(void *unused)
> >  		of_node_put(np);
> >  	}
> >  	of_node_put(l2_cache);
> > -	ipi_call_unlock();
> > 
> >  	local_irq_enable();
> > 
> > -- 
> > 1.7.5.4
> > 
> 
> --
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