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From: Doug Rabson <dfr@nlsystems.com>
To: Poul-Henning Kamp <phk@phk.freebsd.dk>
In-Reply-To: <53418.1086773585@critter.freebsd.dk>
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Subject: Re: cvs commit: src/sys/kern kern_proc.c
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On Wed, 2004-06-09 at 10:33, Poul-Henning Kamp wrote:
> In message <200406090929.i599T8h6065944@repoman.freebsd.org>, Poul-Henning Kamp
>  writes:
> >
> >  Modified files:
> >    sys/kern             kern_proc.c 
> >  Log:
> >  Fix a race in destruction of sessions.
> 
> Not to pick on anybody, but this is a perfect example of getting locking
> almost right:
> 
> BAD:
> 
> 	LOCK(foo->lock)
> 	foo->refcount--;
> 	UNLOCK(foo->lock)
> 	if (foo->refcount == 0)
> 		destroy(foo);
> 
> GOOD:
> 
> 	LOCK(foo->lock)
> 	i = --foo->refcount;
> 	UNLOCK(foo->lock)
> 	if (i == 0)
> 		destroy(foo);

Isn't there still a race in the GOOD case here if somone takes a new
reference, incrementing refcount after the UNLOCK(foo->lock)?