Re: [RFC] how to fix is_local_disk()?

[Andrew Morton]

Christoph Hellwig <[e-mail]> wrote:
>
> ...
> +static void do_emergency_remount(unsigned long foo)
> +{
> +	struct super_block *sb;
> +
> +	spin_lock(&sb_lock);
> +	list_for_each_entry(sb, &super_blocks, s_list) {
> +		sb->s_count++;
> +		spin_unlock(&sb_lock);
> +		down_read(&sb->s_umount);
> +		if (sb->s_bdev && !(sb->s_flags & MS_RDONLY))
> +			do_remount_sb(sb, MS_RDONLY, NULL, 1);
> +		drop_super(sb);

You might need to check sb->s_root in here after acquiring sb->s_umount. 
Otherwise the fs may have been unmounted while the semaphore was being waited
upon.

About half of the s_umount grabbers perform that check.  The others might be
buggy.  I'm not sure - it's all rather gunky in there and hard to tell what
the rules are.