Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Subject: Re: [PATCH -next v7 5/5] md: protect md_thread with rcu
To:     Yu Kuai <yukuai1@huaweicloud.com>, song@kernel.org,
        logang@deltatee.com
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <961691d0-7224-caad-6c19-d9c8ca07a801@deltatee.com>
 <20230414013246.1969224-1-yukuai1@huaweicloud.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <623c9eb9-b8e6-381f-894f-8d7b955e9651@huaweicloud.com>
Date:   Sun, 23 Apr 2023 10:42:10 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20230414013246.1969224-1-yukuai1@huaweicloud.com>
Content-Type: text/plain; charset=gbk; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

Hi,

?? 2023/04/14 9:32, Yu Kuai д??:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Our test reports a uaf for 'mddev->sync_thread':
> 
> T1                      T2
> md_start_sync
>   md_register_thread
>   // mddev->sync_thread is set
> 			raid1d
> 			 md_check_recovery
> 			  md_reap_sync_thread
> 			   md_unregister_thread
> 			    kfree
> 
>   md_wakeup_thread
>    wake_up
>    ->sync_thread was freed
> 
> Root cause is that there is a small windown between register thread and
> wake up thread, where the thread can be freed concurrently.
> 
> Currently, a global spinlock 'pers_lock' is borrowed to protect
> 'mddev->thread', this problem can be fixed likewise, however, there are
> similar problems elsewhere, and use a global lock for all the cases is
> not good.
> 
> This patch protect all md_thread with rcu.

Friendly ping... Or do I need to resend the whole patchset for v7?

Thanks,
Kuai
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>   drivers/md/md-bitmap.c    | 10 ++++--
>   drivers/md/md-cluster.c   | 17 ++++++----
>   drivers/md/md-multipath.c |  4 +--
>   drivers/md/md.c           | 69 ++++++++++++++++++---------------------
>   drivers/md/md.h           |  8 ++---
>   drivers/md/raid1.c        |  7 ++--
>   drivers/md/raid1.h        |  2 +-
>   drivers/md/raid10.c       | 21 +++++++-----
>   drivers/md/raid10.h       |  2 +-
>   drivers/md/raid5-cache.c  | 22 ++++++++-----
>   drivers/md/raid5.c        | 15 +++++----
>   drivers/md/raid5.h        |  2 +-
>   12 files changed, 98 insertions(+), 81 deletions(-)
> 
> diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
> index 29fd41ef55a6..ab27f66dbb1f 100644
> --- a/drivers/md/md-bitmap.c
> +++ b/drivers/md/md-bitmap.c
> @@ -1221,13 +1221,19 @@ static bitmap_counter_t *md_bitmap_get_counter(struct bitmap_counts *bitmap,
>   static void mddev_set_timeout(struct mddev *mddev, unsigned long timeout,
>   			      bool force)
>   {
> -	struct md_thread *thread = mddev->thread;
> +	struct md_thread *thread;
> +
> +	rcu_read_lock();
> +	thread = rcu_dereference(mddev->thread);
>   
>   	if (!thread)
> -		return;
> +		goto out;
>   
>   	if (force || thread->timeout < MAX_SCHEDULE_TIMEOUT)
>   		thread->timeout = timeout;
> +
> +out:
> +	rcu_read_unlock();
>   }
>   
>   /*
> diff --git a/drivers/md/md-cluster.c b/drivers/md/md-cluster.c
> index 10e0c5381d01..bd2e0c61b2e6 100644
> --- a/drivers/md/md-cluster.c
> +++ b/drivers/md/md-cluster.c
> @@ -75,14 +75,14 @@ struct md_cluster_info {
>   	sector_t suspend_hi;
>   	int suspend_from; /* the slot which broadcast suspend_lo/hi */
>   
> -	struct md_thread *recovery_thread;
> +	struct md_thread __rcu *recovery_thread;
>   	unsigned long recovery_map;
>   	/* communication loc resources */
>   	struct dlm_lock_resource *ack_lockres;
>   	struct dlm_lock_resource *message_lockres;
>   	struct dlm_lock_resource *token_lockres;
>   	struct dlm_lock_resource *no_new_dev_lockres;
> -	struct md_thread *recv_thread;
> +	struct md_thread __rcu *recv_thread;
>   	struct completion newdisk_completion;
>   	wait_queue_head_t wait;
>   	unsigned long state;
> @@ -362,8 +362,8 @@ static void __recover_slot(struct mddev *mddev, int slot)
>   
>   	set_bit(slot, &cinfo->recovery_map);
>   	if (!cinfo->recovery_thread) {
> -		cinfo->recovery_thread = md_register_thread(recover_bitmaps,
> -				mddev, "recover");
> +		rcu_assign_pointer(cinfo->recovery_thread,
> +			md_register_thread(recover_bitmaps, mddev, "recover"));
>   		if (!cinfo->recovery_thread) {
>   			pr_warn("md-cluster: Could not create recovery thread\n");
>   			return;
> @@ -526,11 +526,15 @@ static void process_add_new_disk(struct mddev *mddev, struct cluster_msg *cmsg)
>   static void process_metadata_update(struct mddev *mddev, struct cluster_msg *msg)
>   {
>   	int got_lock = 0;
> +	struct md_thread *thread;
>   	struct md_cluster_info *cinfo = mddev->cluster_info;
>   	mddev->good_device_nr = le32_to_cpu(msg->raid_slot);
>   
>   	dlm_lock_sync(cinfo->no_new_dev_lockres, DLM_LOCK_CR);
> -	wait_event(mddev->thread->wqueue,
> +
> +	/* demaon thread must exist */
> +	thread = rcu_dereference_protected(mddev->thread, true);
> +	wait_event(thread->wqueue,
>   		   (got_lock = mddev_trylock(mddev)) ||
>   		    test_bit(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD, &cinfo->state));
>   	md_reload_sb(mddev, mddev->good_device_nr);
> @@ -889,7 +893,8 @@ static int join(struct mddev *mddev, int nodes)
>   	}
>   	/* Initiate the communication resources */
>   	ret = -ENOMEM;
> -	cinfo->recv_thread = md_register_thread(recv_daemon, mddev, "cluster_recv");
> +	rcu_assign_pointer(cinfo->recv_thread,
> +			md_register_thread(recv_daemon, mddev, "cluster_recv"));
>   	if (!cinfo->recv_thread) {
>   		pr_err("md-cluster: cannot allocate memory for recv_thread!\n");
>   		goto err;
> diff --git a/drivers/md/md-multipath.c b/drivers/md/md-multipath.c
> index 66edf5e72bd6..92c45be203d7 100644
> --- a/drivers/md/md-multipath.c
> +++ b/drivers/md/md-multipath.c
> @@ -400,8 +400,8 @@ static int multipath_run (struct mddev *mddev)
>   	if (ret)
>   		goto out_free_conf;
>   
> -	mddev->thread = md_register_thread(multipathd, mddev,
> -					   "multipath");
> +	rcu_assign_pointer(mddev->thread,
> +			   md_register_thread(multipathd, mddev, "multipath"));
>   	if (!mddev->thread)
>   		goto out_free_conf;
>   
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index e3c30500bd15..f680eccf4197 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -70,11 +70,7 @@
>   #include "md-bitmap.h"
>   #include "md-cluster.h"
>   
> -/* pers_list is a list of registered personalities protected
> - * by pers_lock.
> - * pers_lock does extra service to protect accesses to
> - * mddev->thread when the mutex cannot be held.
> - */
> +/* pers_list is a list of registered personalities protected by pers_lock. */
>   static LIST_HEAD(pers_list);
>   static DEFINE_SPINLOCK(pers_lock);
>   
> @@ -92,7 +88,7 @@ static struct workqueue_struct *md_rdev_misc_wq;
>   static int remove_and_add_spares(struct mddev *mddev,
>   				 struct md_rdev *this);
>   static void mddev_detach(struct mddev *mddev);
> -static void md_wakeup_thread_directly(struct md_thread *thread);
> +static void md_wakeup_thread_directly(struct md_thread __rcu *thread);
>   
>   enum md_ro_state {
>   	MD_RDWR,
> @@ -458,8 +454,10 @@ static void md_submit_bio(struct bio *bio)
>    */
>   void mddev_suspend(struct mddev *mddev)
>   {
> -	WARN_ON_ONCE(mddev->thread && current == mddev->thread->tsk);
> -	lockdep_assert_held(&mddev->reconfig_mutex);
> +	struct md_thread *thread = rcu_dereference_protected(mddev->thread,
> +			lockdep_is_held(&mddev->reconfig_mutex));
> +
> +	WARN_ON_ONCE(thread && current == thread->tsk);
>   	if (mddev->suspended++)
>   		return;
>   	wake_up(&mddev->sb_wait);
> @@ -801,13 +799,8 @@ void mddev_unlock(struct mddev *mddev)
>   	} else
>   		mutex_unlock(&mddev->reconfig_mutex);
>   
> -	/* As we've dropped the mutex we need a spinlock to
> -	 * make sure the thread doesn't disappear
> -	 */
> -	spin_lock(&pers_lock);
>   	md_wakeup_thread(mddev->thread);
>   	wake_up(&mddev->sb_wait);
> -	spin_unlock(&pers_lock);
>   }
>   EXPORT_SYMBOL_GPL(mddev_unlock);
>   
> @@ -7891,19 +7884,29 @@ static int md_thread(void *arg)
>   	return 0;
>   }
>   
> -static void md_wakeup_thread_directly(struct md_thread *thread)
> +static void md_wakeup_thread_directly(struct md_thread __rcu *thread)
>   {
> -	if (thread)
> -		wake_up_process(thread->tsk);
> +	struct md_thread *t;
> +
> +	rcu_read_lock();
> +	t = rcu_dereference(thread);
> +	if (t)
> +		wake_up_process(t->tsk);
> +	rcu_read_unlock();
>   }
>   
> -void md_wakeup_thread(struct md_thread *thread)
> +void md_wakeup_thread(struct md_thread __rcu *thread)
>   {
> -	if (thread) {
> -		pr_debug("md: waking up MD thread %s.\n", thread->tsk->comm);
> -		set_bit(THREAD_WAKEUP, &thread->flags);
> -		wake_up(&thread->wqueue);
> +	struct md_thread *t;
> +
> +	rcu_read_lock();
> +	t = rcu_dereference(thread);
> +	if (t) {
> +		pr_debug("md: waking up MD thread %s.\n", t->tsk->comm);
> +		set_bit(THREAD_WAKEUP, &t->flags);
> +		wake_up(&t->wqueue);
>   	}
> +	rcu_read_unlock();
>   }
>   EXPORT_SYMBOL(md_wakeup_thread);
>   
> @@ -7933,22 +7936,15 @@ struct md_thread *md_register_thread(void (*run) (struct md_thread *),
>   }
>   EXPORT_SYMBOL(md_register_thread);
>   
> -void md_unregister_thread(struct md_thread **threadp)
> +void md_unregister_thread(struct md_thread __rcu **threadp)
>   {
> -	struct md_thread *thread;
> +	struct md_thread *thread = rcu_dereference_protected(*threadp, true);
>   
> -	/*
> -	 * Locking ensures that mddev_unlock does not wake_up a
> -	 * non-existent thread
> -	 */
> -	spin_lock(&pers_lock);
> -	thread = *threadp;
> -	if (!thread) {
> -		spin_unlock(&pers_lock);
> +	if (!thread)
>   		return;
> -	}
> -	*threadp = NULL;
> -	spin_unlock(&pers_lock);
> +
> +	rcu_assign_pointer(*threadp, NULL);
> +	synchronize_rcu();
>   
>   	pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
>   	kthread_stop(thread->tsk);
> @@ -9210,9 +9206,8 @@ static void md_start_sync(struct work_struct *ws)
>   {
>   	struct mddev *mddev = container_of(ws, struct mddev, del_work);
>   
> -	mddev->sync_thread = md_register_thread(md_do_sync,
> -						mddev,
> -						"resync");
> +	rcu_assign_pointer(mddev->sync_thread,
> +			   md_register_thread(md_do_sync, mddev, "resync"));
>   	if (!mddev->sync_thread) {
>   		pr_warn("%s: could not start resync thread...\n",
>   			mdname(mddev));
> diff --git a/drivers/md/md.h b/drivers/md/md.h
> index e148e3c83b0d..324558c3fa06 100644
> --- a/drivers/md/md.h
> +++ b/drivers/md/md.h
> @@ -367,8 +367,8 @@ struct mddev {
>   	int				new_chunk_sectors;
>   	int				reshape_backwards;
>   
> -	struct md_thread		*thread;	/* management thread */
> -	struct md_thread		*sync_thread;	/* doing resync or reconstruct */
> +	struct md_thread __rcu		*thread;	/* management thread */
> +	struct md_thread __rcu		*sync_thread;	/* doing resync or reconstruct */
>   
>   	/* 'last_sync_action' is initialized to "none".  It is set when a
>   	 * sync operation (i.e "data-check", "requested-resync", "resync",
> @@ -734,8 +734,8 @@ extern struct md_thread *md_register_thread(
>   	void (*run)(struct md_thread *thread),
>   	struct mddev *mddev,
>   	const char *name);
> -extern void md_unregister_thread(struct md_thread **threadp);
> -extern void md_wakeup_thread(struct md_thread *thread);
> +extern void md_unregister_thread(struct md_thread __rcu **threadp);
> +extern void md_wakeup_thread(struct md_thread __rcu *thread);
>   extern void md_check_recovery(struct mddev *mddev);
>   extern void md_reap_sync_thread(struct mddev *mddev);
>   extern int mddev_init_writes_pending(struct mddev *mddev);
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 68a9e2d9985b..2f1011ffdf09 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -3084,7 +3084,8 @@ static struct r1conf *setup_conf(struct mddev *mddev)
>   	}
>   
>   	err = -ENOMEM;
> -	conf->thread = md_register_thread(raid1d, mddev, "raid1");
> +	rcu_assign_pointer(conf->thread,
> +			   md_register_thread(raid1d, mddev, "raid1"));
>   	if (!conf->thread)
>   		goto abort;
>   
> @@ -3177,8 +3178,8 @@ static int raid1_run(struct mddev *mddev)
>   	/*
>   	 * Ok, everything is just fine now
>   	 */
> -	mddev->thread = conf->thread;
> -	conf->thread = NULL;
> +	rcu_assign_pointer(mddev->thread, conf->thread);
> +	rcu_assign_pointer(conf->thread, NULL);
>   	mddev->private = conf;
>   	set_bit(MD_FAILFAST_SUPPORTED, &mddev->flags);
>   
> diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
> index ebb6788820e7..468f189da7a0 100644
> --- a/drivers/md/raid1.h
> +++ b/drivers/md/raid1.h
> @@ -130,7 +130,7 @@ struct r1conf {
>   	/* When taking over an array from a different personality, we store
>   	 * the new thread here until we fully activate the array.
>   	 */
> -	struct md_thread	*thread;
> +	struct md_thread __rcu	*thread;
>   
>   	/* Keep track of cluster resync window to send to other
>   	 * nodes.
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 6c66357f92f5..6590aa49598c 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -980,6 +980,7 @@ static void lower_barrier(struct r10conf *conf)
>   static bool stop_waiting_barrier(struct r10conf *conf)
>   {
>   	struct bio_list *bio_list = current->bio_list;
> +	struct md_thread *thread;
>   
>   	/* barrier is dropped */
>   	if (!conf->barrier)
> @@ -995,8 +996,11 @@ static bool stop_waiting_barrier(struct r10conf *conf)
>   	    (!bio_list_empty(&bio_list[0]) || !bio_list_empty(&bio_list[1])))
>   		return true;
>   
> +	/* daemon thread must exist while handling io */
> +	thread = rcu_dereference_protected(conf->mddev->thread, true);
> +
>   	/* move on if recovery thread is blocked by us */
> -	if (conf->mddev->thread->tsk == current &&
> +	if (thread->tsk == current &&
>   	    test_bit(MD_RECOVERY_RUNNING, &conf->mddev->recovery) &&
>   	    conf->nr_queued > 0)
>   		return true;
> @@ -4078,7 +4082,8 @@ static struct r10conf *setup_conf(struct mddev *mddev)
>   	atomic_set(&conf->nr_pending, 0);
>   
>   	err = -ENOMEM;
> -	conf->thread = md_register_thread(raid10d, mddev, "raid10");
> +	rcu_assign_pointer(conf->thread,
> +			   md_register_thread(raid10d, mddev, "raid10"));
>   	if (!conf->thread)
>   		goto out;
>   
> @@ -4141,8 +4146,8 @@ static int raid10_run(struct mddev *mddev)
>   		}
>   	}
>   
> -	mddev->thread = conf->thread;
> -	conf->thread = NULL;
> +	rcu_assign_pointer(mddev->thread, conf->thread);
> +	rcu_assign_pointer(conf->thread, NULL);
>   
>   	if (mddev->queue) {
>   		blk_queue_max_write_zeroes_sectors(mddev->queue, 0);
> @@ -4273,8 +4278,8 @@ static int raid10_run(struct mddev *mddev)
>   		clear_bit(MD_RECOVERY_CHECK, &mddev->recovery);
>   		set_bit(MD_RECOVERY_RESHAPE, &mddev->recovery);
>   		set_bit(MD_RECOVERY_RUNNING, &mddev->recovery);
> -		mddev->sync_thread = md_register_thread(md_do_sync, mddev,
> -							"reshape");
> +		rcu_assign_pointer(mddev->sync_thread,
> +			md_register_thread(md_do_sync, mddev, "reshape"));
>   		if (!mddev->sync_thread)
>   			goto out_free_conf;
>   	}
> @@ -4686,8 +4691,8 @@ static int raid10_start_reshape(struct mddev *mddev)
>   	set_bit(MD_RECOVERY_RESHAPE, &mddev->recovery);
>   	set_bit(MD_RECOVERY_RUNNING, &mddev->recovery);
>   
> -	mddev->sync_thread = md_register_thread(md_do_sync, mddev,
> -						"reshape");
> +	rcu_assign_pointer(mddev->sync_thread,
> +			   md_register_thread(md_do_sync, mddev, "reshape"));
>   	if (!mddev->sync_thread) {
>   		ret = -EAGAIN;
>   		goto abort;
> diff --git a/drivers/md/raid10.h b/drivers/md/raid10.h
> index 8c072ce0bc54..63e48b11b552 100644
> --- a/drivers/md/raid10.h
> +++ b/drivers/md/raid10.h
> @@ -100,7 +100,7 @@ struct r10conf {
>   	/* When taking over an array from a different personality, we store
>   	 * the new thread here until we fully activate the array.
>   	 */
> -	struct md_thread	*thread;
> +	struct md_thread __rcu	*thread;
>   
>   	/*
>   	 * Keep track of cluster resync window to send to other nodes.
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index 46182b955aef..040f5d6e1298 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -120,7 +120,7 @@ struct r5l_log {
>   	struct bio_set bs;
>   	mempool_t meta_pool;
>   
> -	struct md_thread *reclaim_thread;
> +	struct md_thread __rcu *reclaim_thread;
>   	unsigned long reclaim_target;	/* number of space that need to be
>   					 * reclaimed.  if it's 0, reclaim spaces
>   					 * used by io_units which are in
> @@ -1576,17 +1576,18 @@ void r5l_wake_reclaim(struct r5l_log *log, sector_t space)
>   
>   void r5l_quiesce(struct r5l_log *log, int quiesce)
>   {
> -	struct mddev *mddev;
> +	struct mddev *mddev = log->rdev->mddev;
> +	struct md_thread *thread = rcu_dereference_protected(
> +		log->reclaim_thread, lockdep_is_held(&mddev->reconfig_mutex));
>   
>   	if (quiesce) {
>   		/* make sure r5l_write_super_and_discard_space exits */
> -		mddev = log->rdev->mddev;
>   		wake_up(&mddev->sb_wait);
> -		kthread_park(log->reclaim_thread->tsk);
> +		kthread_park(thread->tsk);
>   		r5l_wake_reclaim(log, MaxSector);
>   		r5l_do_reclaim(log);
>   	} else
> -		kthread_unpark(log->reclaim_thread->tsk);
> +		kthread_unpark(thread->tsk);
>   }
>   
>   bool r5l_log_disk_error(struct r5conf *conf)
> @@ -3063,6 +3064,7 @@ void r5c_update_on_rdev_error(struct mddev *mddev, struct md_rdev *rdev)
>   int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
>   {
>   	struct r5l_log *log;
> +	struct md_thread *thread;
>   	int ret;
>   
>   	pr_debug("md/raid:%s: using device %pg as journal\n",
> @@ -3121,11 +3123,13 @@ int r5l_init_log(struct r5conf *conf, struct md_rdev *rdev)
>   	spin_lock_init(&log->tree_lock);
>   	INIT_RADIX_TREE(&log->big_stripe_tree, GFP_NOWAIT | __GFP_NOWARN);
>   
> -	log->reclaim_thread = md_register_thread(r5l_reclaim_thread,
> -						 log->rdev->mddev, "reclaim");
> -	if (!log->reclaim_thread)
> +	thread = md_register_thread(r5l_reclaim_thread, log->rdev->mddev,
> +				    "reclaim");
> +	if (!thread)
>   		goto reclaim_thread;
> -	log->reclaim_thread->timeout = R5C_RECLAIM_WAKEUP_INTERVAL;
> +
> +	thread->timeout = R5C_RECLAIM_WAKEUP_INTERVAL;
> +	rcu_assign_pointer(log->reclaim_thread, thread);
>   
>   	init_waitqueue_head(&log->iounit_wait);
>   
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 7b820b81d8c2..1f68bba9d0b9 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7665,7 +7665,8 @@ static struct r5conf *setup_conf(struct mddev *mddev)
>   	}
>   
>   	sprintf(pers_name, "raid%d", mddev->new_level);
> -	conf->thread = md_register_thread(raid5d, mddev, pers_name);
> +	rcu_assign_pointer(conf->thread,
> +			   md_register_thread(raid5d, mddev, pers_name));
>   	if (!conf->thread) {
>   		pr_warn("md/raid:%s: couldn't allocate thread.\n",
>   			mdname(mddev));
> @@ -7889,8 +7890,8 @@ static int raid5_run(struct mddev *mddev)
>   	}
>   
>   	conf->min_offset_diff = min_offset_diff;
> -	mddev->thread = conf->thread;
> -	conf->thread = NULL;
> +	rcu_assign_pointer(mddev->thread, conf->thread);
> +	rcu_assign_pointer(conf->thread, NULL);
>   	mddev->private = conf;
>   
>   	for (i = 0; i < conf->raid_disks && conf->previous_raid_disks;
> @@ -7989,8 +7990,8 @@ static int raid5_run(struct mddev *mddev)
>   		clear_bit(MD_RECOVERY_CHECK, &mddev->recovery);
>   		set_bit(MD_RECOVERY_RESHAPE, &mddev->recovery);
>   		set_bit(MD_RECOVERY_RUNNING, &mddev->recovery);
> -		mddev->sync_thread = md_register_thread(md_do_sync, mddev,
> -							"reshape");
> +		rcu_assign_pointer(mddev->sync_thread,
> +			md_register_thread(md_do_sync, mddev, "reshape"));
>   		if (!mddev->sync_thread)
>   			goto abort;
>   	}
> @@ -8567,8 +8568,8 @@ static int raid5_start_reshape(struct mddev *mddev)
>   	clear_bit(MD_RECOVERY_DONE, &mddev->recovery);
>   	set_bit(MD_RECOVERY_RESHAPE, &mddev->recovery);
>   	set_bit(MD_RECOVERY_RUNNING, &mddev->recovery);
> -	mddev->sync_thread = md_register_thread(md_do_sync, mddev,
> -						"reshape");
> +	rcu_assign_pointer(mddev->sync_thread,
> +			   md_register_thread(md_do_sync, mddev, "reshape"));
>   	if (!mddev->sync_thread) {
>   		mddev->recovery = 0;
>   		spin_lock_irq(&conf->device_lock);
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index e873938a6125..f19707189a7b 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -679,7 +679,7 @@ struct r5conf {
>   	/* When taking over an array from a different personality, we store
>   	 * the new thread here until we fully activate the array.
>   	 */
> -	struct md_thread	*thread;
> +	struct md_thread __rcu	*thread;
>   	struct list_head	temp_inactive_list[NR_STRIPE_HASH_LOCKS];
>   	struct r5worker_group	*worker_groups;
>   	int			group_cnt;
>