Date: Thu, 22 Dec 2011 13:28:41 -0500 From: John Baldwin <jhb@freebsd.org> To: "Dag-Erling =?utf-8?q?Sm=C3=B8rgrav?=" <des@des.no> Cc: Poul-Henning Kamp <phk@phk.freebsd.dk>, freebsd-threads@freebsd.org, freebsd-arch@freebsd.org Subject: Re: [Patch] C1X threading support Message-ID: <201112221328.41221.jhb@freebsd.org> In-Reply-To: <86zkeksftq.fsf@ds4.des.no> References: <73233.1324389741@critter.freebsd.dk> <201112211028.26780.jhb@freebsd.org> <86zkeksftq.fsf@ds4.des.no>
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On Thursday, December 22, 2011 11:59:13 am Dag-Erling Smørgrav wrote:
> John Baldwin <jhb@freebsd.org> writes:
> > Dag-Erling Smørgrav <des@des.no> writes:
> > > Poul-Henning Kamp <phk@phk.freebsd.dk> writes:
> > > > mtx_unlock(l)
> > > > {
> > > > assert(l->id == thread_id);
> > > > l->id = 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"
You can do the same check as mtx_assert_held() internally and fail the unlock
request in that case (or abort or what have you).
> > > > mtx_assert_held(l)
> > > > {
> > > > assert(l->lock-field != 0);
> > > > assert(l->id == 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.
I do think these have to be atomic. I think lock-field must be able to be
atomically read by definition. I think it is not an unreasonable requirement
to have the implementation implement a thread_id that fits in an atomic type
if it is not able to encode the thread_id into the lock cookie itself.
I do think if l->id was not atomic it might be feasible to see a value while
the thread is not locked that is equivalent to the current thread's ID based
on observing different parts of l->id from different writes. There might be
ways the implementation could guard against that however. In practice though
I think pointers are atomic with regards to loads and stores on nearly all
machines.
--
John Baldwin
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