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From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no>
To: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
References: <73233.1324389741@critter.freebsd.dk>
Date: Wed, 21 Dec 2011 15:58:57 +0100
In-Reply-To: <73233.1324389741@critter.freebsd.dk> (Poul-Henning Kamp's
	message of "Tue, 20 Dec 2011 14:02:21 +0000")
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Cc: freebsd-arch@freebsd.org, freebsd-threads@freebsd.org
Subject: Re: [Patch] C1X threading support
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"Poul-Henning Kamp" <phk@phk.freebsd.dk> writes:
> There is no way this can be impossible on a platform which can
> implement a mutex in the first place:
>
>
> 	mtx_lock(l)
> 	{
> 		atomic_magic_lock(l->lock_field)
> 		l->id =3D thread_id;
> 	}

OK

> 	mtx_unlock(l)
> 	{
> 		assert(l->id =3D=3D thread_id);
> 		l->id =3D NULL;
> 		atomic_magic_unlock(l->lock_field)
> 	}

susceptible to race conditions

> 	mtx_assert_held(l)
> 	{
> 		assert(l->lock-field !=3D 0);
> 		assert(l->id =3D=3D thread_id);
> 	}

susceptible to race conditions

The canonical solution is to use some low-level lock primitive (worst
case, a critical section) to protect the mutex structure, but then you
need some way for mtx_lock() to sleep if the mutex is held and some way
for mtx_unlock() to wake sleepers.  You also need to lock the mutex
structure in mtx_assert_held(), unless l->id is atomic.

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