Re: [PATCH] fs,block: get holder during claim

[Jan Kara <jack@suse.cz>]

On Fri 15-03-24 14:23:07, Christian Brauner wrote:
> Now that we open block devices as files we need to deal with the
> realities that closing is a deferred operation. An operation on the
> block device such as e.g., freeze, thaw, or removal that runs
> concurrently with umount, tries to acquire a stable reference on the
> holder. The holder might already be gone though. Make that reliable by
> grabbing a passive reference to the holder during bdev_open() and
> releasing it during bdev_release().
> 
> Fixes: f3a608827d1f ("bdev: open block device as files") # mainline only
> Reported-by: Christoph Hellwig <hch@infradead.org>
> Link: https://lore.kernel.org/r/ZfEQQ9jZZVes0WCZ@infradead.org
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> Hey all,
> 
> I ran blktests with nbd enabled which contains a reliable repro for the
> issue. Thanks to Christoph for pointing in that direction. The
> underlying issue is not reproducible anymore with this patch applied.
> xfstests and blktests pass.

Thanks for the fix! It looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza


> diff --git a/block/bdev.c b/block/bdev.c
> index e7adaaf1c219..7a5f611c3d2e 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -583,6 +583,9 @@ static void bd_finish_claiming(struct block_device *bdev, void *holder,
>  	mutex_unlock(&bdev->bd_holder_lock);
>  	bd_clear_claiming(whole, holder);
>  	mutex_unlock(&bdev_lock);
> +
> +	if (hops && hops->get_holder)
> +		hops->get_holder(holder);
>  }
>  
>  /**
> @@ -605,6 +608,7 @@ EXPORT_SYMBOL(bd_abort_claiming);
>  static void bd_end_claim(struct block_device *bdev, void *holder)
>  {
>  	struct block_device *whole = bdev_whole(bdev);
> +	const struct blk_holder_ops *hops = bdev->bd_holder_ops;
>  	bool unblock = false;
>  
>  	/*
> @@ -627,6 +631,9 @@ static void bd_end_claim(struct block_device *bdev, void *holder)
>  		whole->bd_holder = NULL;
>  	mutex_unlock(&bdev_lock);
>  
> +	if (hops && hops->put_holder)
> +		hops->put_holder(holder);
> +
>  	/*
>  	 * If this was the last claim, remove holder link and unblock evpoll if
>  	 * it was a write holder.
> diff --git a/fs/super.c b/fs/super.c
> index ee05ab6b37e7..71d9779c42b1 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1515,11 +1515,29 @@ static int fs_bdev_thaw(struct block_device *bdev)
>  	return error;
>  }
>  
> +static void fs_bdev_super_get(void *data)
> +{
> +	struct super_block *sb = data;
> +
> +	spin_lock(&sb_lock);
> +	sb->s_count++;
> +	spin_unlock(&sb_lock);
> +}
> +
> +static void fs_bdev_super_put(void *data)
> +{
> +	struct super_block *sb = data;
> +
> +	put_super(sb);
> +}
> +
>  const struct blk_holder_ops fs_holder_ops = {
>  	.mark_dead		= fs_bdev_mark_dead,
>  	.sync			= fs_bdev_sync,
>  	.freeze			= fs_bdev_freeze,
>  	.thaw			= fs_bdev_thaw,
> +	.get_holder		= fs_bdev_super_get,
> +	.put_holder		= fs_bdev_super_put,
>  };
>  EXPORT_SYMBOL_GPL(fs_holder_ops);
>  
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index f9b87c39cab0..c3e8f7cf96be 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1505,6 +1505,16 @@ struct blk_holder_ops {
>  	 * Thaw the file system mounted on the block device.
>  	 */
>  	int (*thaw)(struct block_device *bdev);
> +
> +	/*
> +	 * If needed, get a reference to the holder.
> +	 */
> +	void (*get_holder)(void *holder);
> +
> +	/*
> +	 * Release the holder.
> +	 */
> +	void (*put_holder)(void *holder);
>  };
>  
>  /*
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR