Re: [PATCH 4/9] fsfreeze: emergency thaw will deadlock on s_umount

[Jan Kara <jack@suse.cz>]

On Fri 14-09-12 15:48:56, Fernando Luis Vázquez Cao wrote:
> The emergency thaw process uses iterate_super() which holds the
> sb->s_umount lock in read mode. The current thaw_super() code takes
> the sb->s_umount lock in write mode, hence leading to an instant
> deadlock.
> 
> Use the unlocked version of thaw_super() to do the thawing and replace
> iterate_supers() with __iterate_supers() so that the unfreeze operation can
> be performed with s_umount held as the locking rules for fsfreeze indicate.
> 
> As a bonus, by using thaw_super(), which does not nest, instead of thaw_bdev()
> when can get rid of the ugly while loop.
  Hum, but that will leave block devices frozen? Isn't that a bug? It
definitely creates an inconsistent state. I think emergency thaw should
also iterate over all block devices (via iterate_bdevs()) and thaw all of
them (and no, this does not allow us to avoid the iterate_super() changes
because there are filesystems without block devices...).

								Honza

> 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/buffer.c linux-3.6-rc5/fs/buffer.c
> --- linux-3.6-rc5-orig/fs/buffer.c	2012-09-14 12:46:22.988871880 +0900
> +++ linux-3.6-rc5/fs/buffer.c	2012-09-14 13:05:47.812919103 +0900
> @@ -513,15 +513,27 @@ repeat:
>  
>  static void do_thaw_one(struct super_block *sb, void *unused)
>  {
> -	char b[BDEVNAME_SIZE];
> -	while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
> -		printk(KERN_WARNING "Emergency Thaw on %s\n",
> +	int res;
> +
> +	if (sb->s_bdev) {
> +		char b[BDEVNAME_SIZE];
> +		printk(KERN_WARNING "Emergency Thaw on %s.\n",
>  		       bdevname(sb->s_bdev, b));
> +	}
> +
> +	/* We were called from __iterate_supers with superblock lock taken
> +	 * so we do not need to do it here. */
> +	res = __thaw_super(sb);
> +	if (!res)
> +		deactivate_locked_super(sb);
> +	else
> +		up_write(&sb->s_umount);
> +	return res;
>  }
>  
>  static void do_thaw_all(struct work_struct *work)
>  {
> -	iterate_supers_read(do_thaw_one, NULL);
> +	iterate_supers_write(do_thaw_one, NULL);
>  	kfree(work);
>  	printk(KERN_WARNING "Emergency Thaw complete\n");
>  }
> 
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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