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Date:	Thu, 6 Feb 2014 12:11:51 +1100
From:	NeilBrown <neilb@...e.de>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 45/51] md, raid5: Fix CPU hotplug callback registration

On Thu, 06 Feb 2014 03:42:45 +0530 "Srivatsa S. Bhat"
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:

> From: Oleg Nesterov <oleg@...hat.com>
> 
> Subsystems that want to register CPU hotplug callbacks, as well as perform
> initialization for the CPUs that are already online, often do it as shown
> below:
> 
> 	get_online_cpus();
> 
> 	for_each_online_cpu(cpu)
> 		init_cpu(cpu);
> 
> 	register_cpu_notifier(&foobar_cpu_notifier);
> 
> 	put_online_cpus();
> 
> This is wrong, since it is prone to ABBA deadlocks involving the
> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
> with CPU hotplug operations).
> 
> Interestingly, the raid5 code can actually prevent double initialization and
> hence can use the following simplified form of callback registration:
> 
> 	register_cpu_notifier(&foobar_cpu_notifier);
> 
> 	get_online_cpus();
> 
> 	for_each_online_cpu(cpu)
> 		init_cpu(cpu);
> 
> 	put_online_cpus();
> 
> A hotplug operation that occurs between registering the notifier and calling
> get_online_cpus(), won't disrupt anything, because the code takes care to
> perform the memory allocations only once.
> 
> So reorganize the code in raid5 this way to fix the deadlock with callback
> registration.
> 
> Cc: Neil Brown <neilb@...e.de>
> Cc: linux-raid@...r.kernel.org
> Cc: stable@...r.kernel.org
> [Srivatsa: Fixed the unregister_cpu_notifier() deadlock, added the
> free_scratch_buffer() helper to condense code further and wrote the changelog.]
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> ---
> 
>  drivers/md/raid5.c |   90 +++++++++++++++++++++++++---------------------------
>  1 file changed, 44 insertions(+), 46 deletions(-)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index f1feade..16f5c21 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -5514,23 +5514,43 @@ raid5_size(struct mddev *mddev, sector_t sectors, int raid_disks)
>  	return sectors * (raid_disks - conf->max_degraded);
>  }
>  
> +static void free_scratch_buffer(struct r5conf *conf, struct raid5_percpu *percpu)
> +{
> +	safe_put_page(percpu->spare_page);
> +	kfree(percpu->scribble);
> +	percpu->spare_page = NULL;
> +	percpu->scribble = NULL;
> +}
> +
> +static int alloc_scratch_buffer(struct r5conf *conf, struct raid5_percpu *percpu)
> +{
> +	if (conf->level == 6 && !percpu->spare_page)
> +		percpu->spare_page = alloc_page(GFP_KERNEL);
> +	if (!percpu->scribble)
> +		percpu->scribble = kmalloc(conf->scribble_len, GFP_KERNEL);
> +
> +	if (!percpu->scribble || (conf->level == 6 && !percpu->spare_page)) {
> +		free_scratch_buffer(conf, percpu);
> +		return -ENOMEM;
> +	}
> +
> +	return 0;
> +}
> +
>  static void raid5_free_percpu(struct r5conf *conf)
>  {
> -	struct raid5_percpu *percpu;
>  	unsigned long cpu;
>  
>  	if (!conf->percpu)
>  		return;
>  
> -	get_online_cpus();
> -	for_each_possible_cpu(cpu) {
> -		percpu = per_cpu_ptr(conf->percpu, cpu);
> -		safe_put_page(percpu->spare_page);
> -		kfree(percpu->scribble);
> -	}
>  #ifdef CONFIG_HOTPLUG_CPU
>  	unregister_cpu_notifier(&conf->cpu_notify);
>  #endif
> +
> +	get_online_cpus();
> +	for_each_possible_cpu(cpu)
> +		free_scratch_buffer(conf, per_cpu_ptr(conf->percpu, cpu));
>  	put_online_cpus();
>  
>  	free_percpu(conf->percpu);
> @@ -5557,15 +5577,7 @@ static int raid456_cpu_notify(struct notifier_block *nfb, unsigned long action,
>  	switch (action) {
>  	case CPU_UP_PREPARE:
>  	case CPU_UP_PREPARE_FROZEN:
> -		if (conf->level == 6 && !percpu->spare_page)
> -			percpu->spare_page = alloc_page(GFP_KERNEL);
> -		if (!percpu->scribble)
> -			percpu->scribble = kmalloc(conf->scribble_len, GFP_KERNEL);
> -
> -		if (!percpu->scribble ||
> -		    (conf->level == 6 && !percpu->spare_page)) {
> -			safe_put_page(percpu->spare_page);
> -			kfree(percpu->scribble);
> +		if (alloc_scratch_buffer(conf, percpu)) {
>  			pr_err("%s: failed memory allocation for cpu%ld\n",
>  			       __func__, cpu);
>  			return notifier_from_errno(-ENOMEM);
> @@ -5573,10 +5585,7 @@ static int raid456_cpu_notify(struct notifier_block *nfb, unsigned long action,
>  		break;
>  	case CPU_DEAD:
>  	case CPU_DEAD_FROZEN:
> -		safe_put_page(percpu->spare_page);
> -		kfree(percpu->scribble);
> -		percpu->spare_page = NULL;
> -		percpu->scribble = NULL;
> +		free_scratch_buffer(conf, per_cpu_ptr(conf->percpu, cpu));
>  		break;
>  	default:
>  		break;
> @@ -5588,40 +5597,29 @@ static int raid456_cpu_notify(struct notifier_block *nfb, unsigned long action,
>  static int raid5_alloc_percpu(struct r5conf *conf)
>  {
>  	unsigned long cpu;
> -	struct page *spare_page;
> -	struct raid5_percpu __percpu *allcpus;
> -	void *scribble;
> -	int err;
> +	int err = 0;
>  
> -	allcpus = alloc_percpu(struct raid5_percpu);
> -	if (!allcpus)
> +	conf->percpu = alloc_percpu(struct raid5_percpu);
> +	if (!conf->percpu)
>  		return -ENOMEM;
> -	conf->percpu = allcpus;
> +
> +#ifdef CONFIG_HOTPLUG_CPU
> +	conf->cpu_notify.notifier_call = raid456_cpu_notify;
> +	conf->cpu_notify.priority = 0;
> +	err = register_cpu_notifier(&conf->cpu_notify);
> +	if (err)
> +		return err;
> +#endif
>  
>  	get_online_cpus();
> -	err = 0;
>  	for_each_present_cpu(cpu) {
> -		if (conf->level == 6) {
> -			spare_page = alloc_page(GFP_KERNEL);
> -			if (!spare_page) {
> -				err = -ENOMEM;
> -				break;
> -			}
> -			per_cpu_ptr(conf->percpu, cpu)->spare_page = spare_page;
> -		}
> -		scribble = kmalloc(conf->scribble_len, GFP_KERNEL);
> -		if (!scribble) {
> -			err = -ENOMEM;
> +		err = alloc_scratch_buffer(conf, per_cpu_ptr(conf->percpu, cpu));
> +		if (err) {
> +			pr_err("%s: failed memory allocation for cpu%ld\n",
> +			       __func__, cpu);
>  			break;
>  		}
> -		per_cpu_ptr(conf->percpu, cpu)->scribble = scribble;
>  	}
> -#ifdef CONFIG_HOTPLUG_CPU
> -	conf->cpu_notify.notifier_call = raid456_cpu_notify;
> -	conf->cpu_notify.priority = 0;
> -	if (err == 0)
> -		err = register_cpu_notifier(&conf->cpu_notify);
> -#endif
>  	put_online_cpus();
>  
>  	return err;


Looks good, thanks.
Shall I wait for a signed-of-by from Oleg, then queue it through my md tree?

NeilBrown

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