linux-kernel - Re: [patch 1/9] procfs: Move /proc/pid/fd[info] handling code to fd.[ch]

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Date:	Sat, 25 Aug 2012 22:52:39 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	aneesh.kumar@...ux.vnet.ibm.com, bfields@...ldses.org
Subject: Re: [patch 1/9] procfs: Move /proc/pid/fd[info] handling code to
 fd.[ch]

On Sat, Aug 25, 2012 at 11:43:25PM +0400, Cyrill Gorcunov wrote:

> Would the patch below improve the code? Look, I've not dropped
> find_inode_number call since it's a bit unclear for me what
> would happen if !child case hit
> 
> 	child = d_lookup(dir, &qname);
> 	if (!child) {
> 		struct dentry *new = d_alloc(dir, &qname);
> 		if (new) {
> 			child = instantiate(dir->d_inode, new, task, ptr);
> 			if (child)
> 				dput(new);
> 			else
> 				child = new;
> 		}
> 	}
> 
> can we be sure that i_ino won't be zero here?

First of all, ->i_ino is not going to be zero for any procfs inode.
As for !child case, that's possible only if d_lookup() returns NULL
*and* d_alloc() fails.  In that case find_inode_number() will call
d_hash_and_lookup(), which will call d_lookup(), get NULL from it and
return NULL to find_inode_number().  Which will return 0 to the
caller.

AFAICS, if find_inode_number() is called there at all, it will return 0.
IOW, this if (!ino) ino = find_inode_number(...); is a no-op.
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