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

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Message-ID: <20120921224912.GA20960@dastard>
Date:	Sat, 22 Sep 2012 08:49:12 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Guo Chao <yan@...ux.vnet.ibm.com>
Cc:	viro@...iv.linux.org.uk, dchinner@...hat.com, hch@...radead.org,
	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v4 Patch 0/4] fs/inode.c: optimization for inode lock usage

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@...morbit.com
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