linux-kernel - Re: slab: start_cpu_timer/cache_reap CONFIG_HOTPLUG

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Date:	Tue, 30 Jan 2007 01:14:08 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Christoph Lameter <clameter@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, Srivatsa Vaddagiri <vatsa@...ibm.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Gautham shenoy <ego@...ibm.com>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org
Subject: Re: slab: start_cpu_timer/cache_reap CONFIG_HOTPLUG_CPU problems

On 01/29, Christoph Lameter wrote:
>
> On Tue, 30 Jan 2007, Oleg Nesterov wrote:
> 
> > Now we have 2 additional events, CPU_LOCK_ACQUIRE/CPU_LOCK_RELEASE,
> > so cpuup_callback() can use them to lock/unlock cache_chain_mutex,
> > but this is not related.
> 
> Then we wont need to do the mutex_lock/unlock in CPU_DOWN_XX 
> anymore, right? Which brings us to this form of the patch:

Yes, if CPU_LOCK_ACQUIRE takes cache_chain_mutex, we don't need to do so
on CPU_DOWN_PREPARE. I didn't know cpuup_callback() was already converted,
sorry for the confusion!

Note: with the patch below we are doing cancel_rearming_delayed_work()
under cache_chain_mutex. This is ok since cache_reap() does mutex_trylock(),
so deadlock is not possible. However, this means that mutex_trylock() becomes
mandatory for cache_reap(), probably a little comment will be good.

> Shutdown cache_reaper when cpu goes down
> 
> Shutdown the cache_reaper in slab.c if the cpu is brought down
> and set the cache_reap.func to NULL. Otherwise hotplug shuts
> down the reaper for good.
> 
> Signed-off-by: Christoph Lameter <clameter@....com>
> 
> Index: linux-2.6.20-rc6-mm2/mm/slab.c
> ===================================================================
> --- linux-2.6.20-rc6-mm2.orig/mm/slab.c	2007-01-29 14:27:34.199229828 -0600
> +++ linux-2.6.20-rc6-mm2/mm/slab.c	2007-01-29 15:47:18.293962726 -0600
> @@ -1271,6 +1271,14 @@ static int __cpuinit cpuup_callback(stru
>  		start_cpu_timer(cpu);
>  		break;
>  #ifdef CONFIG_HOTPLUG_CPU
> +  	case CPU_DOWN_PREPARE:
> +		/* Shutdown cache reaper */
> +		cancel_rearming_delayed_work(&per_cpu(reap_work, cpu));
> +		per_cpu(reap_work, cpu).work.func = NULL;
> +  		break;
> +  	case CPU_DOWN_FAILED:
> +		start_cpu_timer(cpu);
> +  		break;
>  	case CPU_DEAD:
>  		/*
>  		 * Even if all the cpus of a node are down, we don't free the

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