Commit d7439fb1 authored by Jan Kara's avatar Jan Kara Committed by Christian Brauner
Browse files

fs: Provide helpers for manipulating sb->s_readonly_remount 



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: default avatarDave Chinner <david@fromorbit.com>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarDave Chinner <dchinner@redhat.com>
Message-Id: <20230620112832.5158-1-jack@suse.cz>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent c541dce8

fs/internal.h

+41 −0
Original line number Diff line number Diff line
@@ -120,6 +120,47 @@ 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();

}



/*

 * 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)

{

	/*

	 * This barrier provides release semantics that pairs 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.

	 */

	smp_wmb();

	WRITE_ONCE(sb->s_readonly_remount, 0);

}



/*

 * open.c

 */

fs/namespace.c

+16 −9
Original line number Diff line number Diff line
@@ -309,9 +309,16 @@ static unsigned int mnt_get_writers(struct mount *mnt) 


static int mnt_is_readonly(struct vfsmount *mnt)

{

	if (mnt->mnt_sb->s_readonly_remount)

	if (READ_ONCE(mnt->mnt_sb->s_readonly_remount))

		return 1;

	/* Order wrt setting s_flags/s_readonly_remount in do_remount() */

	/*

	 * The barrier pairs with the barrier in sb_start_ro_state_change()

	 * making sure if we don't see s_readonly_remount set yet, we also will

	 * not see any superblock / mount flag changes done by remount.

	 * It also pairs with the barrier in sb_end_ro_state_change()

	 * assuring that if we see s_readonly_remount already cleared, we will

	 * see the values of superblock / mount flags updated by remount.

	 */

	smp_rmb();

	return __mnt_is_readonly(mnt);

}
@@ -364,9 +371,11 @@ int __mnt_want_write(struct vfsmount *m) 
		}

	}

	/*

	 * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will

	 * be set to match its requirements. So we must not load that until

	 * MNT_WRITE_HOLD is cleared.

	 * The barrier pairs with the barrier sb_start_ro_state_change() making

	 * sure that if we see MNT_WRITE_HOLD cleared, we will also see

	 * s_readonly_remount set (or even SB_RDONLY / MNT_READONLY flags) in

	 * mnt_is_readonly() and bail in case we are racing with remount

	 * read-only.

	 */

	smp_rmb();

	if (mnt_is_readonly(m)) {
@@ -588,10 +597,8 @@ int sb_prepare_remount_readonly(struct super_block *sb) 
	if (!err && atomic_long_read(&sb->s_remove_count))

		err = -EBUSY;



	if (!err) {

		sb->s_readonly_remount = 1;

		smp_wmb();

	}

	if (!err)

		sb_start_ro_state_change(sb);

	list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {

		if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)

			mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;

fs/super.c

+6 −11
Original line number Diff line number Diff line
@@ -944,8 +944,7 @@ int reconfigure_super(struct fs_context *fc) 
	 */

	if (remount_ro) {

		if (force) {

			sb->s_readonly_remount = 1;

			smp_wmb();

			sb_start_ro_state_change(sb);

		} else {

			retval = sb_prepare_remount_readonly(sb);

			if (retval)
@@ -953,12 +952,10 @@ int reconfigure_super(struct fs_context *fc) 
		}

	} else if (remount_rw) {

		/*

		 * We set s_readonly_remount here to protect filesystem's

		 * reconfigure code from writes from userspace until

		 * reconfigure finishes.

		 * Protect filesystem's reconfigure code from writes from

		 * userspace until reconfigure finishes.

		 */

		sb->s_readonly_remount = 1;

		smp_wmb();

		sb_start_ro_state_change(sb);

	}



	if (fc->ops->reconfigure) {
@@ -974,9 +971,7 @@ int reconfigure_super(struct fs_context *fc) 


	WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |

				 (fc->sb_flags & fc->sb_flags_mask)));

	/* Needs to be ordered wrt mnt_is_readonly() */

	smp_wmb();

	sb->s_readonly_remount = 0;

	sb_end_ro_state_change(sb);



	/*

	 * Some filesystems modify their metadata via some other path than the
@@ -991,7 +986,7 @@ int reconfigure_super(struct fs_context *fc) 
	return 0;



cancel_readonly:

	sb->s_readonly_remount = 0;

	sb_end_ro_state_change(sb);

	return retval;

}

include/linux/fs.h

+1 −1
Original line number Diff line number Diff line
@@ -1248,7 +1248,7 @@ struct super_block { 
	 */

	atomic_long_t s_fsnotify_connectors;



	/* Being remounted read-only */

	/* Read-only state of the superblock is being changed */

	int s_readonly_remount;



	/* per-sb errseq_t for reporting writeback errors via syncfs */