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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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