Re: [PATCH] fs: reintroduce freezing nesting

[Mateusz Guzik <mguzik@redhat.com>]

On Tue, Apr 19, 2016 at 10:48:41PM +0200, Florian Margaine wrote:
> The behavior was removed in 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2
> noting that this was a better idea than using a counter. However, this
> behavior is actually wanted if multiple applications want to freeze
> concurrently while remaining non-racy.
> 
> This patch reintroduces this feature by using a counter.
> 

This patch is wrong.

It uses non-atomic ops to modify the counter and no locks are
held to protect it.

I would argue the code should track that freezing has started and
additional freezers must only return when the state is
SB_FREEZE_COMPLETE.

> ---
>  fs/super.c         | 15 +++++++++++----
>  include/linux/fs.h |  1 +
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 74914b1..9fa8ca1 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -231,6 +231,7 @@ static struct super_block *alloc_super(struct
> file_system_type *type, int flags)
>  	 */
>  	down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
>  	s->s_count = 1;
> +	s->s_freezers = 0;
>  	atomic_set(&s->s_active, 1);
>  	mutex_init(&s->s_vfs_rename_mutex);
>  	lockdep_set_class(&s->s_vfs_rename_mutex, &type-
> >s_vfs_rename_key);
> @@ -1275,12 +1276,12 @@ int freeze_super(struct super_block *sb)
>  {
>  	int ret;
>  
> +	sb->s_freezers++;
> +	if (sb->s_writers.frozen != SB_UNFROZEN)
> +		return 0;
> +
>  	atomic_inc(&sb->s_active);
>  	down_write(&sb->s_umount);
> -	if (sb->s_writers.frozen != SB_UNFROZEN) {
> -		deactivate_locked_super(sb);
> -		return -EBUSY;
> -	}
>  
>  	if (!(sb->s_flags & MS_BORN)) {
>  		up_write(&sb->s_umount);
> @@ -1338,14 +1339,20 @@ EXPORT_SYMBOL(freeze_super);
>   * @sb: the super to thaw
>   *
>   * Unlocks the filesystem and marks it writeable again after
> freeze_super().
> + * Since nesting freezes is allowed, only the last freeze actually
> unlocks.
>   */
>  int thaw_super(struct super_block *sb)
>  {
>  	int error;
>  
> +	sb->s_freezers--;
> +	if (sb->s_freezers > 0)
> +		return 0;
> +
>  	down_write(&sb->s_umount);
>  	if (sb->s_writers.frozen == SB_UNFROZEN) {
>  		up_write(&sb->s_umount);
> +		sb->s_freezers++;
>  		return -EINVAL;
>  	}
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e514f76..c045e2a 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1333,6 +1333,7 @@ struct super_block {
>  	struct quota_info	s_dquot;	/* Diskquota specific
> options */
>  
>  	struct sb_writers	s_writers;
> +	int			s_freezers;
>  
>  	char s_id[32];				/* Informational
> name */
>  	u8 s_uuid[16];				/* UUID */
> -- 
> 2.8.0
> 
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-- 
Mateusz Guzik