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From: John Baldwin <jhb@freebsd.org>
To: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
Date: Tue, 20 Dec 2011 09:43:18 -0500
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Cc: freebsd-threads@freebsd.org, freebsd-arch@freebsd.org
Subject: Re: [Patch] C1X threading support
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On Tuesday, December 20, 2011 9:02:21 am Poul-Henning Kamp wrote:
> In message <201112200822.26369.jhb@freebsd.org>, John Baldwin writes:
> 
> >The reason I can think of why you might not specify 
> >this is if you want to support machines that have very limited support for 
> >atomic operations (e.g. only an exchange instruction or a single-bit test-and-
> >set as opposed to a full-world test-and-set such as cmpxchg on x86 or cas on 
> >sparc).
> 
> 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 = thread_id;
> 	}
> 
> 	mtx_unlock(l)
> 	{
> 		assert(l->id == thread_id);
> 		l->id = NULL;
> 		atomic_magic_unlock(l->lock_field)
> 	}
> 
> 	mtx_assert_held(l)
> 	{
> 		assert(l->lock-field != 0);
> 		assert(l->id == thread_id);
> 	}

Yep, having a helper field to track the owner would work fine on such
degenerate platforms.

-- 
John Baldwin