linux-kernel - Re: [PATCH] make iunique use a do/while loop rather than its obscure goto loop

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Date:	Fri, 13 Apr 2007 12:55:12 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	hch@...radead.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] make iunique use a do/while loop rather than its
 obscure goto loop

On Fri, 13 Apr 2007 15:08:38 -0400
Jeff Layton <jlayton@...hat.com> wrote:

> > 
> > ino_t iunique(struct super_block *sb, ino_t max_reserved)
> > {
> > 	static ino_t counter;
> > 	struct inode *inode;
> > 	struct hlist_head * head;
> > 	ino_t res;
> > 
> > 	spin_lock(&inode_lock);
> > 	do {
> > 		if (counter <= max_reserved)
> > 			counter = max_reserved + 1;
> > 		res = counter++;
> > 		head = inode_hashtable + hash(sb, res);
> > 		inode = find_inode_fast(sb, head, res);
> > 	} while (inode != NULL);
> > 	spin_unlock(&inode_lock);
> > 
> > 	return res;
> > }
> > 
> > The counter-vs-max_reserved test can be moved outside the loop, can't it?
> > 
> 
> No. If the counter wraps while we're looping, then we'll need to skip 
> past the "reserved" inode numbers. So we need to check this on every 
> loop iteration.

oh.

(wonders why alpha and s390 use unsigned int for ino_t while everyone
else uses unsigned long)

> We could potentially put that in an "unlikely" if you 
> think that would be better.

Doubt if it'd make much difference.

> > Shouldn't counter be per-sb?
> 
> I doubt it really matters too much, but it could potentially be more 
> efficient to do that, especially after a wraparound on the counter. It 
> might be reasonable to make new_inode use a per-sb counter as well. Do 
> you think it's worth respinning?

Well, that'd be a separate patch.  Sometime, if you're keen.

If that function is ever a performance problem, it'll be an awful
performance problem and we'd need to so something smarter than
a linear search - an idr tree, for example.
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