Re: [RFC 2/6] fs: add iterate_supers_excl() and iterate_supers_reverse_excl()

[Christian Brauner]

On Wed, Mar 26, 2025 at 04:22:16AM -0700, Luis Chamberlain wrote:
> There are use cases where we wish to traverse the superblock list
> but also capture errors, and in which case we want to avoid having
> our callers issue a lock themselves since we can do the locking for
> the callers. Provide a iterate_supers_excl() which calls a function
> with the write lock held. If an error occurs we capture it and
> propagate it.
> 
> Likewise there are use cases where we wish to traverse the superblock
> list but in reverse order. The new iterate_supers_reverse_excl() helpers
> does this but also also captures any errors encountered.
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
>  fs/super.c         | 91 ++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  2 +
>  2 files changed, 93 insertions(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 117bd1bfe09f..9995546cf159 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -945,6 +945,97 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
>  	spin_unlock(&sb_lock);
>  }
>  
> +/**
> + *	iterate_supers_excl - exclusively call func for all active superblocks
> + *	@f: function to call
> + *	@arg: argument to pass to it
> + *
> + *	Scans the superblock list and calls given function, passing it
> + *	locked superblock and given argument. Returns 0 unless an error
> + *	occurred on calling the function on any superblock.
> + */
> +int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg)
> +{
> +	struct super_block *sb, *p = NULL;
> +	int error = 0;
> +
> +	spin_lock(&sb_lock);
> +	list_for_each_entry(sb, &super_blocks, s_list) {
> +		if (hlist_unhashed(&sb->s_instances))
> +			continue;
> +		sb->s_count++;
> +		spin_unlock(&sb_lock);
> +
> +		down_write(&sb->s_umount);
> +		if (sb->s_root && (sb->s_flags & SB_BORN)) {
> +			error = f(sb, arg);
> +			if (error) {
> +				up_write(&sb->s_umount);
> +				spin_lock(&sb_lock);
> +				__put_super(sb);
> +				break;
> +			}
> +		}
> +		up_write(&sb->s_umount);

This is wrong. Both the reverse and the regular iterator need to wait
for the superblock to be born or die:

void iterate_supers_excl(void (*f)(struct super_block *, void *), void *arg)
{
	struct super_block *sb, *p = NULL;

	spin_lock(&sb_lock);
	list_for_each_entry{_reverse}(sb, &super_blocks, s_list) {
		bool locked;

		sb->s_count++;
		spin_unlock(&sb_lock);

		locked = super_lock(sb);
		if (locked) {
			if (sb->s_root)
				f(sb, arg);
			super_unlock(sb);
		}

		spin_lock(&sb_lock);
		if (p)
			__put_super(p);
		p = sb;
	}
	if (p)
		__put_super(p);
	spin_unlock(&sb_lock);
}

> +
> +		spin_lock(&sb_lock);
> +		if (p)
> +			__put_super(p);
> +		p = sb;
> +	}
> +	if (p)
> +		__put_super(p);
> +	spin_unlock(&sb_lock);
> +
> +	return error;
> +}
> +
> +/**
> + *	iterate_supers_reverse_excl - exclusively calls func in reverse order
> + *	@f: function to call
> + *	@arg: argument to pass to it
> + *
> + *	Scans the superblock list and calls given function, passing it
> + *	locked superblock and given argument, in reverse order, and holding
> + *	the s_umount write lock. Returns if an error occurred.
> + */
> +int iterate_supers_reverse_excl(int (*f)(struct super_block *, void *),
> +					 void *arg)
> +{
> +	struct super_block *sb, *p = NULL;
> +	int error = 0;
> +
> +	spin_lock(&sb_lock);
> +	list_for_each_entry_reverse(sb, &super_blocks, s_list) {
> +		if (hlist_unhashed(&sb->s_instances))
> +			continue;
> +		sb->s_count++;
> +		spin_unlock(&sb_lock);
> +
> +		down_write(&sb->s_umount);
> +		if (sb->s_root && (sb->s_flags & SB_BORN)) {
> +			error = f(sb, arg);
> +			if (error) {
> +				up_write(&sb->s_umount);
> +				spin_lock(&sb_lock);
> +				__put_super(sb);
> +				break;
> +			}
> +		}
> +		up_write(&sb->s_umount);
> +
> +		spin_lock(&sb_lock);
> +		if (p)
> +			__put_super(p);
> +		p = sb;
> +	}
> +	if (p)
> +		__put_super(p);
> +	spin_unlock(&sb_lock);
> +
> +	return error;
> +}
> +
>  /**
>   *	iterate_supers_type - call function for superblocks of given type
>   *	@type: fs type
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 1d9a9c557e1a..da17fd74961c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3538,6 +3538,8 @@ extern struct file_system_type *get_fs_type(const char *name);
>  extern void drop_super(struct super_block *sb);
>  extern void drop_super_exclusive(struct super_block *sb);
>  extern void iterate_supers(void (*)(struct super_block *, void *), void *);
> +extern int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg);
> +extern int iterate_supers_reverse_excl(int (*)(struct super_block *, void *), void *);
>  extern void iterate_supers_type(struct file_system_type *,
>  			        void (*)(struct super_block *, void *), void *);
>  
> -- 
> 2.47.2
>