Re: [PATCH 4/17] fsfreeze: emergency thaw will deadlock on s_umount

[Jan Kara <jack@suse.cz>]

On Mon 07-01-13 20:26:12, 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_write() 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.
> 
> 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.
  The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

								Honza
> 
> Cc: linux-fsdevel@vger.kernel.org
> 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>
> Cc: Luiz Capitulino <lcapitulino@redhat.com>
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
> ---
> 
> diff -urNp linux-3.8-rc1-orig/drivers/tty/sysrq.c linux-3.8-rc1/drivers/tty/sysrq.c
> --- linux-3.8-rc1-orig/drivers/tty/sysrq.c	2012-12-25 10:27:40.614737000 +0900
> +++ linux-3.8-rc1/drivers/tty/sysrq.c	2012-12-25 11:40:06.128018000 +0900
> @@ -363,7 +363,6 @@ static struct sysrq_key_op sysrq_moom_op
>  	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
>  };
>  
> -#ifdef CONFIG_BLOCK
>  static void sysrq_handle_thaw(int key)
>  {
>  	emergency_thaw_all();
> @@ -374,7 +373,6 @@ static struct sysrq_key_op sysrq_thaw_op
>  	.action_msg	= "Emergency Thaw of all frozen filesystems",
>  	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
>  };
> -#endif
>  
>  static void sysrq_handle_kill(int key)
>  {
> diff -urNp linux-3.8-rc1-orig/fs/buffer.c linux-3.8-rc1/fs/buffer.c
> --- linux-3.8-rc1-orig/fs/buffer.c	2012-12-25 11:30:38.208018000 +0900
> +++ linux-3.8-rc1/fs/buffer.c	2012-12-25 11:40:06.128018000 +0900
> @@ -512,15 +512,33 @@ 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 (!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);
> +	}
>  }
>  
>  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");
>  }
> diff -urNp linux-3.8-rc1-orig/include/linux/fs.h linux-3.8-rc1/include/linux/fs.h
> --- linux-3.8-rc1-orig/include/linux/fs.h	2012-12-25 11:35:55.488018000 +0900
> +++ linux-3.8-rc1/include/linux/fs.h	2012-12-25 11:40:06.132018000 +0900
> @@ -1881,6 +1881,7 @@ extern int vfs_ustat(dev_t, struct kstat
>  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 void emergency_thaw_all(void);
>  extern bool our_mnt(struct vfsmount *mnt);
>  
>  extern int current_umask(void);
> @@ -2053,7 +2054,6 @@ extern void iterate_bdevs(void (*)(struc
>  extern int sync_blockdev(struct block_device *bdev);
>  extern void kill_bdev(struct block_device *);
>  extern struct super_block *freeze_bdev(struct block_device *);
> -extern void emergency_thaw_all(void);
>  extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
>  extern int fsync_bdev(struct block_device *);
>  #else
> 
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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