Re: [PATCH v2] fs: Provide helpers for manipulating sb->s_readonly_remount

[Dave Chinner <david@fromorbit.com>]

On Mon, Jun 19, 2023 at 01:18:32PM +0200, Jan Kara wrote:
> Provide helpers to set and clear sb->s_readonly_remount including
> appropriate memory barriers. Also use this opportunity to document what
> the barriers pair with and why they are needed.
> 
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/internal.h      | 34 ++++++++++++++++++++++++++++++++++
>  fs/namespace.c     | 10 ++++------
>  fs/super.c         | 17 ++++++-----------
>  include/linux/fs.h |  2 +-
>  4 files changed, 45 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/internal.h b/fs/internal.h
> index bd3b2810a36b..e206eb58bd3e 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -120,6 +120,40 @@ void put_super(struct super_block *sb);
>  extern bool mount_capable(struct fs_context *);
>  int sb_init_dio_done_wq(struct super_block *sb);
>  
> +/*
> + * Prepare superblock for changing its read-only state (i.e., either remount
> + * read-write superblock read-only or vice versa). After this function returns
> + * mnt_is_readonly() will return true for any mount of the superblock if its
> + * caller is able to observe any changes done by the remount. This holds until
> + * sb_end_ro_state_change() is called.
> + */
> +static inline void sb_start_ro_state_change(struct super_block *sb)
> +{
> +	WRITE_ONCE(sb->s_readonly_remount, 1);
> +	/*
> +	 * For RO->RW transition, the barrier pairs with the barrier in
> +	 * mnt_is_readonly() making sure if mnt_is_readonly() sees SB_RDONLY
> +	 * cleared, it will see s_readonly_remount set.
> +	 * For RW->RO transition, the barrier pairs with the barrier in
> +	 * __mnt_want_write() before the mnt_is_readonly() check. The barrier
> +	 * makes sure if __mnt_want_write() sees MNT_WRITE_HOLD already
> +	 * cleared, it will see s_readonly_remount set.
> +	 */
> +	smp_wmb();
> +}

Can you please also update mnt_is_readonly/__mnt_want_write to
indicate that there is a pairing with this helper from those
functions?

> +
> +/*
> + * Ends section changing read-only state of the superblock. After this function
> + * returns if mnt_is_readonly() returns false, the caller will be able to
> + * observe all the changes remount did to the superblock.
> + */
> +static inline void sb_end_ro_state_change(struct super_block *sb)
> +{
> +	/* The barrier pairs with the barrier in mnt_is_readonly() */
> +	smp_wmb();
> +	WRITE_ONCE(sb->s_readonly_remount, 0);
> +}

	/*
	 * This barrier provides release semantics that pair with
	 * the smp_rmb() acquire semantics in mnt_is_readonly().
	 * This barrier pair ensure that when mnt_is_readonly() sees
	 * 0 for sb->s_readonly_remount, it will also see all the
	 * preceding flag changes that were made during the RO state
	 * change.
	 */

And a comment in mnt_is_readonly() to indicate that it also pairs
with sb_end_ro_state_change() in a different way to the barriers in
sb_start_ro_state_change(), __mnt_want_write(), MNT_WRITE_HOLD, etc.

Memory barriers need clear, concise documentation, otherwise they
are impossible to understand from just reading the code...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com