Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)

[Martin Wirth]

Rusty Russell wrote:

>
>The solution I was referring to before, using full semaphores, would
>look like so:
>
>struct pthread_cond_t
>{
>	int num_waiting;
>	struct futex wait, ack;
>};
>
>#define PTHREAD_COND_INITIALIZER { 0, { 0 }, { 0 } }
>
>int pthread_cond_signal(pthread_cond_t *cond)
>{
>	if (cond->num_waiters)
>		return futex_up(&cond->futex, 1);
>	return 0;
>}
>
>int pthread_cond_broadcast(pthread_cond_t *cond)
>{
>	unsigned int waiters = cond->num_waiting;
>
>	if (waiters) {
>		futex_up(&cond->futex, waiters);
>		/* Wait for ack before returning. */
>		futex_down(&cond->ack);
>	}
>	return 0;
>}
>
>int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
>{
>	int ret;
>
>	/* Increment first so broadcaster knows we are waiting. */
>	atomic_inc(cond->num_waiting);
>	futex_up(&mutex, 1);
>	ret = futex_down(&cond);
>	if (atomic_dec_and_test(cond->num_waiting))
>		futex_up(&cond->ack);
>	futex_down(&mutex->futex);
>	return ret;
>}
>
In principle that works. But one of  things that's less nice with 
pthread_cond_wait is
that you sometimes have a (most of the time) unnecessary schedule 
ping-pong, and with the
approach above you always have this (due to ack). And secondly if 
futex_up(&f, N) for N > 1
relies on the chained wakeup in the kernels futex_up routine the 
broadcast may take a while to
complete (the lowest priority waiter penalizes all others queued behind 
him). A semaphore simply is no full replacement for a waitqueue with 
wake_all.

Martin

P.S.  With respect to pthreads I was not thinking of a bloated N:M 
library, but of some simple
fast pthread semantics compatible wrapper for _clone etc.