Re: New USB stack on FreeBSD w/Linux emulation layer coming

Hi,

On Thursday 17 May 2007 19:03, Alan Stern wrote:
> On Thu, 17 May 2007, Hans Petter Selasky wrote:
> > > In Linux, the USB callbacks are generally atomic.  Does that answer
> > > your question?
> >
> > Yes. So you protect the callback with a lock to make it atomic?
>
> No, you don't understand.  The callback is atomic because it is not
> allowed to sleep.  That's what "atomic" means.  It has nothing to do
> with locking or protection.

And it cannot be interrupted either?

>
> > > > b) You don't pre-allocate transfers, resulting in
> > > > synchronous/blocking calls everywhere, and no-one thinks about what
> > > > can happen then :-)
> > >
> > > Your reasoning is illogical.  Why does allocation have to be blocking?
> >
> > Memory allocation is blocking in that way it might have to flush out
> > memory pages to a harddisk, before those pages can be allocated by
> > something in the kernel. Of course there exists no-wait options that you
> > can pass to "malloc", but I wouldn't rely too much on those flags
>
> You _can_ rely on the "no-wait" option to prevent the allocation from
> sleeping or flushing pages to a disk...
>
> >  unless you are allocating from a
> > pre-defined memory pool, where you know that there is memory reserved for
> > you.
>
> ... and you can't rely on "no-wait" to succeed unless, as you say, you
> already know there is memory available.  But that's also true for
> early, blocking allocations; there's nothing special about late,
> non-blocking malloc.
>
> With pre-allocation you might fail at the beginning, and with late
> allocation you might fail in the middle.  Either way, you fail.  So
> what's the big deal?

There is a greater chance of successing with pre-allocation if you use the 
wait flag when you malloc? Right?

--HPS

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