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From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no>
To: John Baldwin <jhb@freebsd.org>
References: <73233.1324389741@critter.freebsd.dk> <86hb0ut1hq.fsf@ds4.des.no>
	<201112211028.26780.jhb@freebsd.org>
Date: Thu, 22 Dec 2011 17:59:13 +0100
In-Reply-To: <201112211028.26780.jhb@freebsd.org> (John Baldwin's message of
	"Wed, 21 Dec 2011 10:28:26 -0500")
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Cc: Poul-Henning Kamp <phk@phk.freebsd.dk>, freebsd-threads@freebsd.org,
	Niall Douglas <s_sourceforge@nedprod.com>, freebsd-arch@freebsd.org
Subject: Re: [Patch] C1X threading support
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John Baldwin <jhb@freebsd.org> writes:
> Dag-Erling Sm=C3=B8rgrav <des@des.no> writes:
> > Poul-Henning Kamp <phk@phk.freebsd.dk> writes:
> > > 	mtx_unlock(l)
> > > 	{
> > > 		assert(l->id =3D=3D thread_id);
> > > 		l->id =3D NULL;
> > > 		atomic_magic_unlock(l->lock_field)
> > > 	}
> > susceptible to race conditions
> How so?

I should have specified "if called from a thread that does not own the
mutex"

> > > 	mtx_assert_held(l)
> > > 	{
> > > 		assert(l->lock-field !=3D 0);
> > > 		assert(l->id =3D=3D thread_id);
> > > 	}
> > susceptible to race conditions
> How so?

I was going to point out that the state of the mutex can change between
the two asserts, but as you say, at least one of them is guaranteed to
fail...  *if* you assume that these fields can be read atomically, which
was one of my objections.

DES
--=20
Dag-Erling Sm=C3=B8rgrav - des@des.no