Date: Tue, 20 Dec 2011 09:43:18 -0500 From: John Baldwin <jhb@freebsd.org> To: "Poul-Henning Kamp" <phk@phk.freebsd.dk> Cc: freebsd-threads@freebsd.org, Niall Douglas <s_sourceforge@nedprod.com>, freebsd-arch@freebsd.org Subject: Re: [Patch] C1X threading support Message-ID: <201112200943.18812.jhb@freebsd.org> In-Reply-To: <73233.1324389741@critter.freebsd.dk> References: <73233.1324389741@critter.freebsd.dk>
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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
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