Re: [RFC] Semaphores used for daemon wakeup

[Daniel Phillips <phillips@innominate.de>]

Paul Cassella wrote:
> >     dmabuf_alloc(...)
> >     {
> >             while (1) {
> >                     spin_lock(&dmabuf_lock);
> >                     attempt to grab a free buffer;
> >                     spin_unlock(&dmabuf_lock);
> >                     if (success)
> >                            return;
> >                     down(&dmabuf_wait);
> >             }
> >     }
> 
> >     dmabuf_free(...)
> >     {
> >             spin_lock(&dmabuf_lock);
> >             free up buffer;
> >             spin_unlock(&dmabuf_lock);
> >             up(&dmabuf_wait);
> >     }
> 
> This does things a little differently than the way the original did it.
> I thought the original implied that dmabuf_free() might free up multiple
> buffers.  There's no indication in the comments that this is the case, but
> the original, by using vall_sema(), wakes up all dmabuf_alloc()'s that had
> gone to sleep.

Granted, it's just an example, but it doesn't make sense to wake up more
dmabuf_alloc waiters than you actually have buffers for.  You do one
up() per freed buffer, and the semaphore's wait queue better be fifo or
have some other way of ensuring a task doesn't languish there.  (I don't
know, does it?)
 
> The example wasn't meant to be an ideal use of sv's, but merely as an
> example of how they could be used to achieve the same behavior as the code
> that was posted.

Yes, and a third example of the 'unlock/wakeup_and_sleep' kind of
primitive - there seems to be a pattern.  I should at least take a look
and see if up_down is easy or hard to implement.

--
Daniel
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