Re: [PATCH 4/8] fs: remove inode_lock from iput_final and prune_icache

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Oct 29, 2010 at 02:23:36PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Now that inode state changes are protected by the inode->i_lock and
> the inode LRU manipulations by the inode_lru_lock, we can remove the
> inode_lock from prune_icache and the initial part of iput_final().
> 
> instead of using the inode_lock to protect the inode during
> iput_final, use the inode->i_lock instead. This protects the inode
> against new references being taken while we change the inode state
> to I_FREEING, as well as preventing prune_icache from grabbing the
> inode while we are manipulating it. Hence we no longer need the
> inode_lock in iput_final prior to setting I_FREEING on the inode.
> 
> For prune_icache, we no longer need the inode_lock to protect the
> LRU list, and the inodes themselves are protected against freeing
> races by the inode->i_lock. Hence we can lift the inode_lock from
> prune_icache as well.

	Careful.  At that point we still rely on inode_lock to protect
inode_unhashed().  Note that ->drop_inode() uses it a lot and this step
moves it from inode_lock to ->i_lock.

	What you need to do is pretty simple - make remove_inode_hash()
take both inode_lock (later - inode_hash_lock) and ->i_lock.  That's
enough for inode_unhashed() protection, but I'd also hold ->i_lock on
insertions into hash.  It's trivial (we hold ->i_lock just next to that
insertion) and would make for more consistent rules.