Re: [PATCH 4/4] fs: rcu protect inode hash lookups

[Christoph Hellwig <hch@infradead.org>]

> -	hlist_add_fake(&inode->i_hash);
> +	hlist_nulls_add_fake(&inode->i_hash);

Please add a preparatory inode_fake_hash/inode_mark_hashed or similar
helper to isolate filesystems from the implementation details of the
hash list.

> +	/*
> +	 * reset the inode number so during RCU traversals we do not match this
> +	 * inode in any lookups until it is fully re-initialised again during
> +	 * allocation.
> +	 */
> +	inode->i_ino = 0;

There is no hard rule that i_ino is an invalid inode number.  It can
happen quite easily for inodes using the generic last_ino allocator,
and I would not be surprised if there's some filesystems using it as
part of the on disk layour either.

> +			rcu_read_unlock();
> +			if (locked)
> +				spin_unlock(&inode_hash_lock);
>  			__wait_on_freeing_inode(inode);
> +			if (locked)
> +				spin_lock(&inode_hash_lock);

I can't say I like the locked argument, but I don't see an easy way
around it.  Can you at least keept the unlocking/relocking inside
__wait_on_freeing_inode so that it's centralized in a single place for
both find_inode pathes?

While at it moving __wait_on_freeing_inode to be above ifind would
making changes in this area a lot easier to read, so maybe you can throw
in a patch for that, too?

>  static struct inode *ifind(struct super_block *sb,
> -		struct hlist_head *head, int (*test)(struct inode *, void *),
> +		struct hlist_nulls_head *head, int chain,
> +		int (*test)(struct inode *, void *),
>  		void *data, const int wait)
>  {
>  	struct inode *inode;
>  
> -	spin_lock(&inode_hash_lock);
> -	inode = find_inode(sb, head, test, data);
> +	inode = find_inode(sb, head, chain, test, data, false);
>  	if (inode) {
> -		spin_unlock(&inode_hash_lock);
>  		if (likely(wait))
>  			wait_on_inode(inode);
>  		return inode;
>  	}
> -	spin_unlock(&inode_hash_lock);
>  	return NULL;
>  }

This is starting to get a rather pointless helper.  I'd suggest just
killing ifind/ifind_fast and opencoding them in the caller, possibly
as a preparatory patch.