Commit 789b5e03 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by NeilBrown
Browse files

md/raid5: Fix CPU hotplug callback registration 



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: linux-raid@vger.kernel.org
Cc: stable@vger.kernel.org (v2.6.32+)
Fixes: 36d1c647
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
[Srivatsa: Fixed the unregister_cpu_notifier() deadlock, added the
free_scratch_buffer() helper to condense code further and wrote the changelog.]
Signed-off-by: default avatarSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: default avatarNeilBrown <neilb@suse.de>
parent 1877db75

drivers/md/raid5.c

+44 −46
Original line number Diff line number Diff line
@@ -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;



	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;

			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);

	if (err)

		return err;

#endif



	get_online_cpus();

	for_each_present_cpu(cpu) {

		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;

		}

	}

	put_online_cpus();



	return err;

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