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

[Jan Kara <jack@suse.cz>]

On Tue 20-06-23 09:25:05, Dave Chinner wrote:
> 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?

Ok, I've expanded / updated the comments there.

> > +
> > +/*
> > + * 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.

Thanks! I've added your comment and expanded more on pairing with
sb_end_ro_state_change().

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR