netdev - Re: [PATCH v2 5/5] fs: new_inode_single() and iput

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Message-ID: <20081129111452.GB11959@logfs.org>
Date:	Sat, 29 Nov 2008 12:14:53 +0100
From:	Jörn Engel <joern@...fs.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Christoph Hellwig <hch@...radead.org>,
	David Miller <davem@...emloft.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	"kernel-testers@...r.kernel.org >> Kernel Testers List" 
	<kernel-testers@...r.kernel.org>, Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH v2 5/5] fs: new_inode_single() and iput_single()

On Sat, 29 November 2008 09:45:09 +0100, Eric Dumazet wrote:
>  
> +void iput_single(struct inode *inode)
> +{
> +	if (atomic_dec_and_test(&inode->i_count)) {
> +		destroy_inode(inode);
> +		percpu_counter_dec(&nr_inodes);
> +	}
> +}

I wonder if it is possible to avoid the atomic_dec_and_test() here, at
least in the common case, and combine it with the atomic_dec_and_test()
of the dentry.  A quick look at fs/inode.c indicates that inode->i_count
may never get changed for a SINGLE inode, except during creation or
deletion.

It might be worth to
- remove the conditional from iput_single() and measure that it makes a
  difference,
- poison SINGLE inodes with some value and
- put a BUG_ON() in __iget() that checks for the poison value.

I _think_ the BUG_ON() is unnecessary, but at least my brain is not
sufficient to convince me.  Can inotify somehow get a hold of a socket?
Or dquot (how insane would that be?)

Jörn

-- 
Mac is for working,
Linux is for Networking,
Windows is for Solitaire!
-- stolen from dc
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