Re: [PATCH] block: fix deadlock between bd_link_disk_holder and partition scan

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

Hi, Christoph

在 2024/02/07 17:27, linan666@xxxxxxxxxxxxxxx 写道:
> From: Li Nan <linan122@xxxxxxxxxx>
>
> 'open_mutex' of gendisk is used to protect open/close block devices. But
> in bd_link_disk_holder(), it is used to protect the creation of symlink
> between holding disk and slave bdev, which introduces some issues.
>
> When bd_link_disk_holder() is called, the driver is usually in the process
> of initialization/modification and may suspend submitting io. At this
> time, any io hold 'open_mutex', such as scanning partitions, can cause
> deadlocks. For example, in raid:
>
> T1                              T2
> bdev_open_by_dev
>   lock open_mutex [1]
>   ...
>    efi_partition
>    ...
>     md_submit_bio
> 				md_ioctl mddev_syspend
> 				  -> suspend all io
> 				 md_add_new_disk
> 				  bind_rdev_to_array
> 				   bd_link_disk_holder
> 				    try lock open_mutex [2]
>      md_handle_request
>       -> wait mddev_resume
>
> T1 scan partition, T2 add a new device to raid. T1 waits for T2 to resume
> mddev, but T2 waits for open_mutex held by T1. Deadlock occurs.
>
> Fix it by introducing a local mutex 'holder_mutex' to replace 'open_mutex'.

Can you take a look at this patch? I think for raid(perhaps and dm and
other drivers), it's reasonable to suspend IO while hot adding new
underlying disks. And I think add new slaves to holder is not related to
open the holder disk, because caller should already open the holder disk
to hot add slaves, hence 'open_mutex' for holder is not necessary here.

Actually bd_link_disk_holder() is protected by 'reconfig_mutex' for
raid, and 'table_devices_lock' for dm(I'm not sure yet if other drivers
have similiar lock).

For raid, we do can fix this problem in raid by delay
bd_link_disk_holder() while the array is not suspended, however, we'll
consider this fix later if you think this patch is not acceptable.

Thanks,
Kuai

> Signed-off-by: Li Nan <linan122@xxxxxxxxxx>
> ---
>   block/holder.c | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/block/holder.c b/block/holder.c
> index 37d18c13d958..5bfb0a674cc7 100644
> --- a/block/holder.c
> +++ b/block/holder.c
> @@ -8,6 +8,8 @@ struct bd_holder_disk {
>   	int			refcnt;
>   };
>   
> +static DEFINE_MUTEX(holder_mutex);
> +
>   static struct bd_holder_disk *bd_find_holder_disk(struct block_device *bdev,
>   						  struct gendisk *disk)
>   {
> @@ -80,7 +82,7 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
>   	kobject_get(bdev->bd_holder_dir);
>   	mutex_unlock(&bdev->bd_disk->open_mutex);
>   
> -	mutex_lock(&disk->open_mutex);
> +	mutex_lock(&holder_mutex);
>   	WARN_ON_ONCE(!bdev->bd_holder);
>   
>   	holder = bd_find_holder_disk(bdev, disk);
> @@ -108,7 +110,7 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
>   		goto out_del_symlink;
>   	list_add(&holder->list, &disk->slave_bdevs);
>   
> -	mutex_unlock(&disk->open_mutex);
> +	mutex_unlock(&holder_mutex);
>   	return 0;
>   
>   out_del_symlink:
> @@ -116,7 +118,7 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
>   out_free_holder:
>   	kfree(holder);
>   out_unlock:
> -	mutex_unlock(&disk->open_mutex);
> +	mutex_unlock(&holder_mutex);
>   	if (ret)
>   		kobject_put(bdev->bd_holder_dir);
>   	return ret;
> @@ -140,7 +142,7 @@ void bd_unlink_disk_holder(struct block_device *bdev, struct gendisk *disk)
>   	if (WARN_ON_ONCE(!disk->slave_dir))
>   		return;
>   
> -	mutex_lock(&disk->open_mutex);
> +	mutex_lock(&holder_mutex);
>   	holder = bd_find_holder_disk(bdev, disk);
>   	if (!WARN_ON_ONCE(holder == NULL) && !--holder->refcnt) {
>   		del_symlink(disk->slave_dir, bdev_kobj(bdev));
> @@ -149,6 +151,6 @@ void bd_unlink_disk_holder(struct block_device *bdev, struct gendisk *disk)
>   		list_del_init(&holder->list);
>   		kfree(holder);
>   	}
> -	mutex_unlock(&disk->open_mutex);
> +	mutex_unlock(&holder_mutex);
>   }
>   EXPORT_SYMBOL_GPL(bd_unlink_disk_holder);