Re: [PATCH 1/9] vfs: add __iterate_supers() and helpers around it

[Jan Kara <jack@suse.cz>]

On Fri 14-09-12 15:45:04, Fernando Luis Vázquez Cao wrote:
> iterate_supers() calls a function provided by the caller with the s_umount
> semaphore taken in read mode. However, there may be cases where write mode
> is preferable, so we add __iterate_supers(), which lets one
> specify the mode of the lock, and replace iterate_supers with two helpers
> around __iterate_supers(), iterate_supers_read() and iterate_supers_write().
> 
> This will be used to fix the emergency thaw (filesystem unfreeze) code, which
> iterates over the list of superblocks but needs to hold the s_umount semaphore
> in _write_ mode bebore carrying out the actual thaw operation.
> 
> This patch introduces no semantic changes since iterate_supers() users become
> iterate_supers_read() which is equivalent.
> 
> 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	2012-09-14 11:53:43.416703312 +0900
> +++ linux-3.6-rc5/fs/super.c	2012-09-14 12:30:52.188833193 +0900
> @@ -537,14 +537,22 @@ void drop_super(struct super_block *sb)
>  EXPORT_SYMBOL(drop_super);
>  
>  /**
> - *	iterate_supers - call function for all active superblocks
> + *	__iterate_supers - call function for all active superblocks
>   *	@f: function to call
>   *	@arg: argument to pass to it
> + *	@wlock: mode of superblock lock (false->read lock, true->write lock)
>   *
>   *	Scans the superblock list and calls given function, passing it
>   *	locked superblock and given argument.
> + *
> + *	When the caller asks for the superblock lock (s_umount semaphore) to be
> + *	taken in write mode, the lock is taken but not released because the
> + *	function provided by the caller may deactivate the superblock itself.
> + *	It is that function's job to unlock the superblock as needed in such a
> + *	case.
>   */
> -void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
> +static void __iterate_supers(void (*f)(struct super_block *, void *), void *arg,
> +			     bool wlock)
>  {
>  	struct super_block *sb, *p = NULL;
>  
> @@ -555,10 +563,19 @@ void iterate_supers(void (*f)(struct sup
>  		sb->s_count++;
>  		spin_unlock(&sb_lock);
>  
> -		down_read(&sb->s_umount);
> +		if (wlock)
> +			down_write(&sb->s_umount);
> +		else
> +			down_read(&sb->s_umount);
> +
>  		if (sb->s_root && (sb->s_flags & MS_BORN))
>  			f(sb, arg);
> -		up_read(&sb->s_umount);
> +
> +		/* When the semaphore was taken in write mode the function
> +		 * provided by the caller takes care of unlocking it as
> +		 * needed. See explanation above for details. */
> +		if (!wlock)
> +			up_read(&sb->s_umount);
>  
>  		spin_lock(&sb_lock);
>  		if (p)
  These locking rules are ugly and counterintuitive. People will easily
get them wrong and create bugs. I'd rather see emergency thaw retake the
s_umount semaphore so that iterate_supers() can drop it...

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