[3.11.y.z extended stable] Patch "md/raid5: Fix CPU hotplug callback registration" has been added to staging queue

[Luis Henriques <luis.henriques@canonical.com>]

This is a note to let you know that I have just added a patch titled

    md/raid5: Fix CPU hotplug callback registration

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

From 2bb9ccc6d7b8f2e536d3a3eec1f3c2cbba3a0cb8 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Thu, 6 Feb 2014 03:42:45 +0530
Subject: md/raid5: Fix CPU hotplug callback registration

commit 789b5e0315284463617e106baad360cb9e8db3ac upstream.

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
Fixes: 36d1c6476be51101778882897b315bd928c8c7b5
Signed-off-by: Oleg 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: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Luis Henriques <luis.henriques@canonical.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 a40b969..f64379b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5037,23 +5037,43 @@ 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);
@@ -5079,15 +5099,7 @@ 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);
@@ -5095,10 +5107,7 @@ 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;
@@ -5110,40 +5119,29 @@ 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;
--
1.9.0