linux-kernel - Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

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Date:	Tue, 30 Jan 2007 08:47:07 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Andrew Morton <akpm@...l.org>,
	dipankar@...ibm.com, Gautham Shenoy <ego@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, vatsa@...ibm.com
Subject: Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

On Tue, Jan 30, 2007 at 07:32:27PM +0530, Gautham R Shenoy wrote:
> On Mon, Jan 29, 2007 at 06:45:49PM -0800, Paul E. McKenney wrote:
> > 
> > 	static int _cpu_down(unsigned int cpu)
> > 	{
> > 		int err;
> > 		struct task_struct *p;
> > 		cpumask_t old_allowed, tmp;
> > 
> > 		if (num_online_cpus() == 1)
> > 			return -EBUSY;
> > 
> > 		if (!cpu_online(cpu))
> > 			return -EINVAL;
> > 
> > 		if (freeze_processes()) {
> > 			err = -EBUSY;
> > 			goto out_freeze_notify_failed;
> > 		}
> > 		err = raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
> > 							(void *)(long)cpu);
> > 		if (err == NOTIFY_BAD) {
> > 			printk("%s: attempt to take down CPU %u failed\n",
> > 					__FUNCTION__, cpu);
> > 			err = -EINVAL;
> > 			goto out_freeze_notify_failed;
> > 		}
> > 
> > 		/* Ensure that we are not runnable on dying cpu */
> > 		old_allowed = current->cpus_allowed;
> > 		tmp = CPU_MASK_ALL;
> > 		cpu_clear(cpu, tmp);
> > 		set_cpus_allowed(current, tmp);
> > 
> > 		mutex_lock(&cpu_bitmask_lock);
>  
> We won't be needing this lock either, since it forms the core of
> the unpopular lock_cpu_hotplug :-)

;-)

> > 		p = __stop_machine_run(take_cpu_down, NULL, cpu);
> > 		mutex_unlock(&cpu_bitmask_lock);
>  
> Looks good. 
>  
> I'm wondering if we'll need to try_to_freeze in the fork and exit code 
> as well :?

Quite possibly -- if you check into that, I will look at PF_NOFREEZE.

Fair enough?

							Thanx, Paul
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