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Subject: Re: [PATCH v2] md: fix duplicate filename for rdev
To:     Paul Menzel <pmenzel@molgen.mpg.de>,
        Yu Kuai <yukuai1@huaweicloud.com>
Cc:     song@kernel.org, neilb@suse.de, akpm@linux-foundation.org,
        linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230522133225.2983667-1-yukuai1@huaweicloud.com>
 <876eb6b4-db0f-02e8-9b24-4db3ebc6962d@molgen.mpg.de>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <06ec466d-398e-5817-7bf9-78b96b3f72e8@huaweicloud.com>
Date:   Tue, 23 May 2023 09:16:15 +0800
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Hi,

在 2023/05/22 22:03, Paul Menzel 写道:
> Dear Yu,
> 
> 
> Thank you for your patch. Just a few nits, that can be ignored.

Thanks for these sugestions, I'll update them in v3.

Kuai
> 
> Am 22.05.23 um 15:32 schrieb Yu Kuai:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> commit 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device
> 
> I’d start with capital letter: Commit ….
> 
>> from an md array via sysfs") delay the deleting of rdev, however, this
> 
> 1.  delay*s*
> 2.  s/deleting/deletion/
> 
>> introduce a window that rdev can be added again while the deleting is
> 
> 1.  introduce*s*
> 2.  s/deleting/deletion/
> 
>> not done yet, and sysfs will complain about duplicate filename.
>>
>> Follow up patches try to fix this problem by flush workqueue, however,
> 
> flush*ing* the work queue
> 
>> flush_rdev_wq() is just dead code, the progress in
>> md_kick_rdev_from_array():
> 
> … is:
> 
>> 1) list_del_rcu(&rdev->same_set);
>> 2) synchronize_rcu();
>> 3) queue_work(md_rdev_misc_wq, &rdev->del_work);
>>
>> So in flush_rdev_wq(), if rdev is found in the list, work_pending() can
>> never pass, in the meantime, if work is queued, then rdev can never be
>> found in the list.
>>
>> flush_rdev_wq() can be replaced by flush_workqueue() directly, however,
>> this approach is not good:
>> - the workqueue is global, this synchronization for all raid disks is
>>    not necessary.
> 
> The work queue …
> 
>> - flush_workqueue can't be called under 'reconfig_mutex', there is still
>>    a small window between flush_workqueue() and mddev_lock() that other
>>    context can queue new work, hence the problem is not solved 
>> completely.
> 
> context*s*
> 
>> sysfs already have apis to support delete itself through writer, and
> 
> 1.  s/have/has/
> 2.  deleting
> 
>> these apis, specifically sysfs_break/unbreak_active_protection(), is used
> 
> s/is/are/
> 
>> to support deleting rdev synchronously. Therefore, the above commit 
>> can be
>> reverted, and sysfs duplicate filename can be avoided.
>>
>> A new mdadm regression test is proposed as well.
> 
> It’s not included, right? Then I’d remove the sentence, or write: … is 
> going to be proposed …
> 
> 
> Kind regards,
> 
> Paul
> 
> 
>> Link: 
>> https://lore.kernel.org/linux-raid/20230428062845.1975462-1-yukuai1@huaweicloud.com/ 
>>
>> Fixes: 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a 
>> device from an md array via sysfs")
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>> ---
>> Changes in v2:
>>   - rebase from the latest md-next branch
>>
>>   drivers/md/md.c | 84 +++++++++++++++++++++++++------------------------
>>   drivers/md/md.h |  8 +++++
>>   2 files changed, 51 insertions(+), 41 deletions(-)
>>
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index 7455bc9d8498..cafb457d614c 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -92,6 +92,7 @@ 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 export_rdev(struct md_rdev *rdev);
>>   /*
>>    * Default number of read corrections we'll attempt on an rdev
>> @@ -643,9 +644,11 @@ void mddev_init(struct mddev *mddev)
>>   {
>>       mutex_init(&mddev->open_mutex);
>>       mutex_init(&mddev->reconfig_mutex);
>> +    mutex_init(&mddev->delete_mutex);
>>       mutex_init(&mddev->bitmap_info.mutex);
>>       INIT_LIST_HEAD(&mddev->disks);
>>       INIT_LIST_HEAD(&mddev->all_mddevs);
>> +    INIT_LIST_HEAD(&mddev->deleting);
>>       timer_setup(&mddev->safemode_timer, md_safemode_timeout, 0);
>>       atomic_set(&mddev->active, 1);
>>       atomic_set(&mddev->openers, 0);
>> @@ -747,6 +750,23 @@ static void mddev_free(struct mddev *mddev)
>>   static const struct attribute_group md_redundancy_group;
>> +static void md_free_rdev(struct mddev *mddev)
>> +{
>> +    struct md_rdev *rdev;
>> +    struct md_rdev *tmp;
>> +
>> +    if (list_empty_careful(&mddev->deleting))
>> +        return;
>> +
>> +    mutex_lock(&mddev->delete_mutex);
>> +    list_for_each_entry_safe(rdev, tmp, &mddev->deleting, same_set) {
>> +        list_del_init(&rdev->same_set);
>> +        kobject_del(&rdev->kobj);
>> +        export_rdev(rdev);
>> +    }
>> +    mutex_unlock(&mddev->delete_mutex);
>> +}
>> +
>>   void mddev_unlock(struct mddev *mddev)
>>   {
>>       if (mddev->to_remove) {
>> @@ -788,6 +808,8 @@ void mddev_unlock(struct mddev *mddev)
>>       } else
>>           mutex_unlock(&mddev->reconfig_mutex);
>> +    md_free_rdev(mddev);
>> +
>>       /* As we've dropped the mutex we need a spinlock to
>>        * make sure the thread doesn't disappear
>>        */
>> @@ -2428,13 +2450,6 @@ static int bind_rdev_to_array(struct md_rdev 
>> *rdev, struct mddev *mddev)
>>       return err;
>>   }
>> -static void rdev_delayed_delete(struct work_struct *ws)
>> -{
>> -    struct md_rdev *rdev = container_of(ws, struct md_rdev, del_work);
>> -    kobject_del(&rdev->kobj);
>> -    kobject_put(&rdev->kobj);
>> -}
>> -
>>   void md_autodetect_dev(dev_t dev);
>>   static void export_rdev(struct md_rdev *rdev)
>> @@ -2452,6 +2467,8 @@ static void export_rdev(struct md_rdev *rdev)
>>   static void md_kick_rdev_from_array(struct md_rdev *rdev)
>>   {
>> +    struct mddev *mddev = rdev->mddev;
>> +
>>       bd_unlink_disk_holder(rdev->bdev, rdev->mddev->gendisk);
>>       list_del_rcu(&rdev->same_set);
>>       pr_debug("md: unbind<%pg>\n", rdev->bdev);
>> @@ -2465,15 +2482,17 @@ static void md_kick_rdev_from_array(struct 
>> md_rdev *rdev)
>>       rdev->sysfs_unack_badblocks = NULL;
>>       rdev->sysfs_badblocks = NULL;
>>       rdev->badblocks.count = 0;
>> -    /* We need to delay this, otherwise we can deadlock when
>> -     * writing to 'remove' to "dev/state".  We also need
>> -     * to delay it due to rcu usage.
>> -     */
>> +
>>       synchronize_rcu();
>> -    INIT_WORK(&rdev->del_work, rdev_delayed_delete);
>> -    kobject_get(&rdev->kobj);
>> -    queue_work(md_rdev_misc_wq, &rdev->del_work);
>> -    export_rdev(rdev);
>> +
>> +    /*
>> +     * kobject_del() will wait for all in progress writers to be 
>> done, where
>> +     * reconfig_mutex is held, hence it can't be called under
>> +     * reconfig_mutex and it's delayed to mddev_unlock().
>> +     */
>> +    mutex_lock(&mddev->delete_mutex);
>> +    list_add(&rdev->same_set, &mddev->deleting);
>> +    mutex_unlock(&mddev->delete_mutex);
>>   }
>>   static void export_array(struct mddev *mddev)
>> @@ -3541,6 +3560,7 @@ rdev_attr_store(struct kobject *kobj, struct 
>> attribute *attr,
>>   {
>>       struct rdev_sysfs_entry *entry = container_of(attr, struct 
>> rdev_sysfs_entry, attr);
>>       struct md_rdev *rdev = container_of(kobj, struct md_rdev, kobj);
>> +    struct kernfs_node *kn = NULL;
>>       ssize_t rv;
>>       struct mddev *mddev = rdev->mddev;
>> @@ -3548,6 +3568,10 @@ rdev_attr_store(struct kobject *kobj, struct 
>> attribute *attr,
>>           return -EIO;
>>       if (!capable(CAP_SYS_ADMIN))
>>           return -EACCES;
>> +
>> +    if (entry->store == state_store && cmd_match(page, "remove"))
>> +        kn = sysfs_break_active_protection(kobj, attr);
>> +
>>       rv = mddev ? mddev_lock(mddev) : -ENODEV;
>>       if (!rv) {
>>           if (rdev->mddev == NULL)
>> @@ -3556,6 +3580,10 @@ rdev_attr_store(struct kobject *kobj, struct 
>> attribute *attr,
>>               rv = entry->store(rdev, page, length);
>>           mddev_unlock(mddev);
>>       }
>> +
>> +    if (kn)
>> +        sysfs_unbreak_active_protection(kn);
>> +
>>       return rv;
>>   }
>> @@ -4479,20 +4507,6 @@ null_show(struct mddev *mddev, char *page)
>>       return -EINVAL;
>>   }
>> -/* need to ensure rdev_delayed_delete() has completed */
>> -static void flush_rdev_wq(struct mddev *mddev)
>> -{
>> -    struct md_rdev *rdev;
>> -
>> -    rcu_read_lock();
>> -    rdev_for_each_rcu(rdev, mddev)
>> -        if (work_pending(&rdev->del_work)) {
>> -            flush_workqueue(md_rdev_misc_wq);
>> -            break;
>> -        }
>> -    rcu_read_unlock();
>> -}
>> -
>>   static ssize_t
>>   new_dev_store(struct mddev *mddev, const char *buf, size_t len)
>>   {
>> @@ -4520,7 +4534,6 @@ new_dev_store(struct mddev *mddev, const char 
>> *buf, size_t len)
>>           minor != MINOR(dev))
>>           return -EOVERFLOW;
>> -    flush_rdev_wq(mddev);
>>       err = mddev_lock(mddev);
>>       if (err)
>>           return err;
>> @@ -5590,7 +5603,6 @@ struct mddev *md_alloc(dev_t dev, char *name)
>>        * removed (mddev_delayed_delete).
>>        */
>>       flush_workqueue(md_misc_wq);
>> -    flush_workqueue(md_rdev_misc_wq);
>>       mutex_lock(&disks_mutex);
>>       mddev = mddev_alloc(dev);
>> @@ -7553,9 +7565,6 @@ static int md_ioctl(struct block_device *bdev, 
>> fmode_t mode,
>>       }
>> -    if (cmd == ADD_NEW_DISK || cmd == HOT_ADD_DISK)
>> -        flush_rdev_wq(mddev);
>> -
>>       if (cmd == HOT_REMOVE_DISK)
>>           /* need to ensure recovery thread has run */
>>           wait_event_interruptible_timeout(mddev->sb_wait,
>> @@ -9618,10 +9627,6 @@ static int __init md_init(void)
>>       if (!md_misc_wq)
>>           goto err_misc_wq;
>> -    md_rdev_misc_wq = alloc_workqueue("md_rdev_misc", 0, 0);
>> -    if (!md_rdev_misc_wq)
>> -        goto err_rdev_misc_wq;
>> -
>>       ret = __register_blkdev(MD_MAJOR, "md", md_probe);
>>       if (ret < 0)
>>           goto err_md;
>> @@ -9640,8 +9645,6 @@ static int __init md_init(void)
>>   err_mdp:
>>       unregister_blkdev(MD_MAJOR, "md");
>>   err_md:
>> -    destroy_workqueue(md_rdev_misc_wq);
>> -err_rdev_misc_wq:
>>       destroy_workqueue(md_misc_wq);
>>   err_misc_wq:
>>       destroy_workqueue(md_wq);
>> @@ -9937,7 +9940,6 @@ static __exit void md_exit(void)
>>       }
>>       spin_unlock(&all_mddevs_lock);
>> -    destroy_workqueue(md_rdev_misc_wq);
>>       destroy_workqueue(md_misc_wq);
>>       destroy_workqueue(md_wq);
>>   }
>> diff --git a/drivers/md/md.h b/drivers/md/md.h
>> index 1eec65cf783c..4d191db831da 100644
>> --- a/drivers/md/md.h
>> +++ b/drivers/md/md.h
>> @@ -531,6 +531,14 @@ struct mddev {
>>       unsigned int            good_device_nr;    /* good device num 
>> within cluster raid */
>>       unsigned int            noio_flag; /* for memalloc scope API */
>> +    /*
>> +     * Temporarily store rdev that will be finally removed when
>> +     * reconfig_mutex is unlocked.
>> +     */
>> +    struct list_head        deleting;
>> +    /* Protect the deleting list */
>> +    struct mutex            delete_mutex;
>> +
>>       bool    has_superblocks:1;
>>       bool    fail_last_dev:1;
>>       bool    serialize_policy:1;
> 
> .
>