Date: Wed, 21 Dec 2011 15:58:57 +0100 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: "Poul-Henning Kamp" <phk@phk.freebsd.dk> Cc: freebsd-arch@freebsd.org, Niall Douglas <s_sourceforge@nedprod.com>, freebsd-threads@freebsd.org Subject: Re: [Patch] C1X threading support Message-ID: <86hb0ut1hq.fsf@ds4.des.no> In-Reply-To: <73233.1324389741@critter.freebsd.dk> (Poul-Henning Kamp's message of "Tue, 20 Dec 2011 14:02:21 %2B0000") References: <73233.1324389741@critter.freebsd.dk>
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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 = thread_id;
> }
OK
> mtx_unlock(l)
> {
> assert(l->id == thread_id);
> l->id = NULL;
> atomic_magic_unlock(l->lock_field)
> }
susceptible to race conditions
> mtx_assert_held(l)
> {
> assert(l->lock-field != 0);
> assert(l->id == 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
--
Dag-Erling Smørgrav - des@des.no
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