Re: [RFC 2/9] fsfreeze: add unlocked version of thaw_super

[Eric Sandeen <sandeen@redhat.com>]

On 9/13/12 6:01 AM, Fernando Luis Vázquez Cao wrote:
> thaw_super may be called with superblock lock already taken (fsfreeze's
> emergency thaw being one example), so we need an unlocked version to avoid
> lockups.
> 
> Cc: Josef Bacik <jbacik@fusionio.com>
> Cc: Eric Sandeen <sandeen@redhat.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
> ---
> 
> diff -urNp linux-3.6-rc5-orig/fs/super.c linux-3.6-rc5/fs/super.c
> --- linux-3.6-rc5-orig/fs/super.c	2012-09-12 19:13:00.710047001 +0900
> +++ linux-3.6-rc5/fs/super.c	2012-09-12 19:13:12.185924077 +0900
> @@ -1420,40 +1420,58 @@ int freeze_super(struct super_block *sb)
>  EXPORT_SYMBOL(freeze_super);
>  
>  /**
> - * thaw_super -- unlock filesystem
> + * __thaw_super -- unlock filesystem
>   * @sb: the super to thaw
>   *
> - * Unlocks the filesystem and marks it writeable again after freeze_super().
> + * Unlocks the filesystem and marks it writeable again.
> + *
> + * This is the unlocked version of thaw_super, so it is the caller's job to
> + * to protect the superblock by grabing the s_umount semaphore in write mode.

s/grabing/grabbing/ - and maybe:

+ * and release it again on return, see thaw_super().

But the logic looks fine,

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

>   */
> -int thaw_super(struct super_block *sb)
> +int __thaw_super(struct super_block *sb)
>  {
> -	int error;
> +	int error = 0;
>  
> -	down_write(&sb->s_umount);
>  	if (sb->s_writers.frozen == SB_UNFROZEN) {
> -		up_write(&sb->s_umount);
> -		return -EINVAL;
> +		error = -EINVAL;
> +		goto out;
>  	}
>  
>  	if (sb->s_flags & MS_RDONLY)
> -		goto out;
> +		goto out_thaw;
>  
>  	if (sb->s_op->unfreeze_fs) {
>  		error = sb->s_op->unfreeze_fs(sb);
>  		if (error) {
>  			printk(KERN_ERR
>  				"VFS:Filesystem thaw failed\n");
> -			up_write(&sb->s_umount);
> -			return error;
> +			goto out;
>  		}
>  	}
>  
> -out:
> +out_thaw:
>  	sb->s_writers.frozen = SB_UNFROZEN;
>  	smp_wmb();
>  	wake_up(&sb->s_writers.wait_unfrozen);
> -	deactivate_locked_super(sb);
> +out:
> +	return error;
> +}
>  
> -	return 0;
> +/**
> + * thaw_super -- unlock filesystem
> + * @sb: the super to thaw
> + *
> + * Unlocks the filesystem and marks it writeable again after freeze_super().
> + */
> +int thaw_super(struct super_block *sb)
> +{
> +	int res;
> +	down_write(&sb->s_umount);
> +	res = __thaw_super(sb);
> +	if (!res)
> +		deactivate_locked_super(sb);
> +	else
> +		up_write(&sb->s_umount);
> +	return res;
>  }
>  EXPORT_SYMBOL(thaw_super);
> diff -urNp linux-3.6-rc5-orig/include/linux/fs.h linux-3.6-rc5/include/linux/fs.h
> --- linux-3.6-rc5-orig/include/linux/fs.h	2012-09-12 19:13:00.830047035 +0900
> +++ linux-3.6-rc5/include/linux/fs.h	2012-09-12 19:13:12.197929820 +0900
> @@ -2082,6 +2082,7 @@ extern int user_statfs(const char __user
>  extern int fd_statfs(int, struct kstatfs *);
>  extern int vfs_ustat(dev_t, struct kstatfs *);
>  extern int freeze_super(struct super_block *super);
> +extern int __thaw_super(struct super_block *super);
>  extern int thaw_super(struct super_block *super);
>  extern bool our_mnt(struct vfsmount *mnt);
>  
> 
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html