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

[Jan Kara <jack@suse.cz>]

On Fri 05-10-12 14:35:53, 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
                        ^^ iterate_supers_write()

> 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.
> 
> Jan Kara pointed out that with this approach we will leave the block devices
> frozen, but this is a problem we have had since the introduction of the
> superblock level API: if we thaw the filesystem using the superblock level API
> (be it through the thaw ioctl or emergency thaw) the bdev level freeze
> reference counter (bd_fsfreeze_count) will not be updated and even though
> subsequent calls to thaw_bdev() will decrease it it will never get back to 0
> (if thaw_super() returns an error, and it will when the superblock is unfrozen,
> thaw_bdev() will return without decreasing the counter). The solution I propose
> (and will be implementing in the followup patch "fsfreeze: freeze_super and
> thaw_bdev don't play well together") is letting bd_fsfreeze_count
> become zero when the superblock sitting on top of it is unfrozen, so that
> future calls to freeze_bdev() actually try to freeze the superblock.
> 
> 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-3.6.0-rc7-orig/fs/buffer.c linux-3.6.0-rc7/fs/buffer.c
> --- linux-3.6.0-rc7-orig/fs/buffer.c	2012-09-26 13:20:14.842365056 +0900
> +++ linux-3.6.0-rc7/fs/buffer.c	2012-09-26 15:02:22.630595704 +0900
> @@ -513,15 +513,28 @@ 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 got here from __iterate_supers with the superblock lock taken
> +	 * so we can call the lockless version of thaw_super() safely. */
> +	res = __thaw_super(sb);
> +	/* If we are going to drop the final active reference call
> +	 * deactivate_locked_super to clean things up. In the general case
> +	 * we avoid calling deactivate_locked_super() because it would relase
> +	 * the superblock lock, which is __iterate_supers()'s job. */
> +	if (!res && !atomic_add_unless(&sb->s_active, -1, 1))
> +		deactivate_locked_super(sb);
  This just looks wrong. When we *do* end up calling
deactivate_locked_super() we will return with sb unlocked which makes
iterate_supers_write() unlock already unlocked lock. What I would put here
is:
	if (!res) {
		deactivate_locked_super(sb);
		/*
		 * We have to re-acquire s_umount because
		 * iterate_supers_write() will unlock it. It still holds
		 * passive reference so sb cannot be freed under us.
		 */
		down_write(&sb->s_umount);
	}
	
Is there any problem with this I miss?

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