Messages in this thread |  | | Date | Fri, 16 Aug 2002 17:30:13 +0100 | From | Jamie Lokier <> | Subject | Re: CLONE_DETACHED and exit notification (was user-vm-unlock-2.5.31-A2) |
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Ingo Molnar wrote: > there are some practical problems with making the notification a futex, > not a simple flag. Eg. futexes right now do not force any lock-counter > format upon userspace. Futexes can be used as mutexes, conditional > variables, read-write locks, all of which have different atomic counter > formats and uses.
Agreed; futexes are lovely because they are so generic.
> By doing the TID-release notification via a futex the actual format of > the lock is forced, which is a cleanliness problem. Just writing $0 to > the TID pointer is a robust thing on the other hand.
Quite. There is no lock; the the futex is used in its purest form, as a wait queue.
TID = thread is alive zero = thread is gone
There's no "lock-counter format", because this isn't a lock -- it's a wakeup. There no need for atomicity either, because the listener only reads, it doesn't write. I'm not sure if a PROT_SEM word is required for cache coherency, but if it is, your current implementation requires one too.
Here's a synchronous thread_join-style waiter; it is architecture-neutral:
while (tid = *tid_address) != 0) retval = sys_futex (tid_address, FUTEX_WAIT, tid, 0);
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