Re: [RFC v4 Patch 0/4] fs/inode.c: optimization for inode lock usage

[Dave Chinner]

On Fri, Sep 21, 2012 at 05:31:02PM +0800, Guo Chao wrote:
> This patchset optimizes several places which take the per inode spin lock.
> They have not been fully tested yet, thus they are marked as RFC. 

Inodes are RCU freed. The i_lock spinlock on the i_state field forms
part of the memory barrier that allows the RCU read side to
correctly detect a freed inode during a RCU protected cache lookup
(hash list traversal for the VFS, or a radix tree traversal for XFS).
The i_lock usage around the hahs list operations ensures the hash
list operations are atomic with state changes so that such changes
are correctly detected during RCU-protected traversals...

IOWs, removing the i_lock from around the i_state transitions and
inode hash insert/remove/traversal operations will cause races in
the RCU lookups and result in incorrectly using freed inodes instead
of failing the lookup and creating a new one.

So I don't think this is a good idea at all...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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