Date: Wed, 09 Jun 2004 17:25:00 +0100 From: Doug Rabson <dfr@nlsystems.com> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/kern kern_proc.c Message-ID: <1086798299.12306.3.camel@builder02.qubesoft.com> In-Reply-To: <53418.1086773585@critter.freebsd.dk> References: <53418.1086773585@critter.freebsd.dk>
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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)?
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