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

Rusty Russell (
Wed, 27 Mar 2002 10:10:22 +1100

In message <> you write:
> >
> > And on top of them:
> > futex_down(struct futex *);
> > futex_up(struct futex *);
> >
> Why not keep the simple one-sys-call interface for the fuxtexes. The
> code is so small that it is
> not worth to delete it.

Because it's a premature optimization. We can add it later if someone
proves it's a major hotspot. But it may turn out some other primitive
is more important: I'm not smart enough to know.

> >int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
> >{
> > futex_down(&mutex);
> >
> You should loop here in order catch signals:
> while ( futex_down(&mutex) < 0 && errno == EINTR)


> > if (ret < 0 && errno == EINTR)
> > goto again;
> >
> This assumes that you are allowed to do a double uwaitq_add.

Oops.... Fixed by moving the uwaitq_add above the again: label.

> >
> > saved_errno = errno;
> > uwaitq_remove(cond);
> > futex_down(&mutex);
> >
> Also loop here


> Now whats interesting is the kernel part. I must admit that I haven't
> fully understood all
> effects of the double use of the cookie in your first implementation.
> But if you use a memory
> location as identifier you have to keep a separate flag within
> uwaitq_head that is zeroed
> before you add to the waitqueue and set by the signal functions. Then
> uwaitq_wait has to check for it.

Yes. My prior implementation actually deleted the sleeper from the
queue on wakeup, making it easy to detect and also saving a syscall.
I'm leaning towards the "flag" idea now though.

> uwaitq_remove also takes an argument, are you heading for waiting on
> multiple events?

Yes. The problem is: how many? Each one takes memory and (as you
point out) pinned pages (although this could be changed with some loss
of simplicity). An arbitrary limit smacks of SysV IPC: a sure sign of
bad taste. So for the moment, the limit is 1.


  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at