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From: John Baldwin <jhb@freebsd.org>
To: Roman Divacky <rdivacky@freebsd.org>
Date: Tue, 15 Aug 2006 11:27:54 -0400
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Cc: Perforce Change Reviews <perforce@freebsd.org>
Subject: Re: PERFORCE change 104029 for review
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On Tuesday 15 August 2006 10:07, Roman Divacky wrote:
> http://perforce.freebsd.org/chv.cgi?CH=104029
> 
> Change 104029 by rdivacky@rdivacky_witten on 2006/08/15 14:06:41
> 
> 	Protect against racing concurent creation in futex_get()
> 	
> 	Pointed out by: jhb
> 
> Affected files ...
> 
> .. //depot/projects/soc2006/rdivacky_linuxolator/compat/linux/linux_futex.c#24 
edit
> 
> Differences ...
> 
> 
==== //depot/projects/soc2006/rdivacky_linuxolator/compat/linux/linux_futex.c#24 
(text+ko) ====
> 
> @@ -342,13 +342,13 @@
>  	if (locked == FUTEX_UNLOCKED)
>     	   	FUTEX_UNLOCK;
>  
> +	if (locked == FUTEX_UNLOCKED)
> +   	   	FUTEX_LOCK;

Looks like you should collapse the lock/unlock.  However, it's probably best 
to use mutexes instead of sx locks, and to instead do something like this:

	lock();
	if (item in list) {
		item->ref++;
		unlock();
		return (item);
	}
	unlock();

	new_item = new_item();

	lock();
	if (item in list) {
		item->ref++;
		unlock();
		free(new_item);
		return (item);
	}
	insert new_item
	new_item->ref++;
	unlock();
	return (new_item)

-- 
John Baldwin