On an MP system there simply is no guarantee of that “very, very fast”
quality. The spinlock busy wait can be deferred indefinately by
interrupts, unless of course the spinlock busy wait is executed at a high
enough interrupt level to prevent such indeterminacy.
On Dec 28, 2007 10:40 AM, Alberto Moreira wrote:
> Preemption here is hardware. An interrupt happens, generates the
> equivalent
> of a forced call to an isr. The isr gets control, does whatever it needs
> to
> do very, very fast, and irets back to the caller, who keeps on spinning.
> And
> the whole point of spinning is exactly that you know that you need to hog
> the processor on that spinning loop, because it isn’t safe to proceed
> otherwise.
>
> And if you use the opportunity to wrestle processor control away from the
> spin loop, you’re engaging in dangerous behavior that can easily lead into
> a
> deadlock or livelock. That’s when that “contract” thing becomes necessary,
> and all hell breaks loose because people want to embellish a barebones
> construct into something it was never meant to be.
>
> The way I see it, and I may be wrong, even when a spinlock loop can be
> preempted by the hardware, it should not be interfered with by the OS.
> Spinlocks should be subarchitectural to the OS: the lowest peel of the
> onion!
>
> Alberto.
>
>
>
> ----- Original Message -----
> From:
> To: “Windows System Software Devs Interest List”
> Sent: Thursday, December 27, 2007 6:27 PM
> Subject: Re: [ntdev] Are callbacks guaranteed to run consecutively?
>
>
> > Preemption is allowed, but not suspension
> >
> > What really is the motive for coming up with such a synch* primitive?
> Why
> > not mutex? Why not event? Why not semaphore with count == 1 ?
> >
> > The motive is not to trap the HW in an useless state. If preemption is
> not
> > allowed, how would the system maintain other higher priority problem(s)
> > and tasks ( clocks, instruction OP code problems, and other stuff ).
> >
> > The context switching is one of the culprit ( heavy over head ) of those
> > suspendable synch primitive. A suspendable synch primitive is one where
> > the acquiring thread could be suspended, and rescheduled, and again
> > suspended …
> >
> > It is definitely this suspension, that motivated spinlock. Now if they
> > are also not preemptible then there is a real problem though.
> >
> > It is altogether a different story, if someone tries to optimize the bus
> > traffic by not polling the flag/lock too often, but that is still
> spinning
> > and not suspending or going to wait state where scheduler/dispatcher -
> > intervention is needed to bring the said thread into execution.
> >
> > -pro
> >> A spinlock is supposed to stall a processor. It’s a strong tool to be
> >> used
> >> as an arbiter of last resort. It works like this:
> >>
> >> top: atomic_test_and_set location,true
> >> jump_if_old_value_is_true top
> >>
> >> Or, in C-like prose,
> >>
> >> do
> >> {
> >> old_value = atomic_test_and_set (flag,true);
> >> }
> >> while (old_value);
> >>
> >> To clear a spinlock is even simpler:
> >>
> >> flag = false;
> >>
> >> There’s no yielding of any sort here. Spinlocks don’t block, don’t
> yield,
> >> and they’re also not aware of processes, threads, or whatever. It may
> be
> >> the case that an interrupt causes a context switch, but once control
> >> comes
> >> back, the loop carries on. And I’m definitely not sold on the wisdom of
> >> allowing certain kinds of interrupts and locks to be preempted.
> >>
> >> You can dress up a spinlock with all sorts of extra baggage, but the
> more
> >> you add, the less it looks like a spinlock!
> >>
> >>
> >> Alberto.
> >>
> >>
> >>
> >>
> >> ----- Original Message -----
> >> From: Mark Roddy
> >> To: Windows System Software Devs Interest List
> >> Sent: Wednesday, December 26, 2007 10:57 AM
> >> Subject: Re: [ntdev] Are callbacks guaranteed to run consecutively?
> >>
> >>
> >>
> >>
> >>
> >> On Dec 26, 2007 9:22 AM, Alberto Moreira wrote:
> >>
> >> I’m sorry, people, I’m going to put on my computer science hat now
> >> and
> >> say that whatever this is, it ain’t a spinlock. The whole point of a
> >> spinlock is to spin without yielding execution!
> >>
> >> At least provide an accurate definition. " The whole point of a
> >> spinlock
> >> is to spin without yielding execution! " No that isn’t the whole
> >> point. That is a distinct feature. And everything we have discussed
> >> here, with the exception of the AIX style ‘spin a while, then sleep’
> >> implementation, doesn’t ‘yield execution’. What we have discussed is
> >> that on NT, and most other modern multiprocessor general purpose
> >> operating systems, spinlocks are interruptible. The currently executing
> >> thread does not yield execution, it is interrupted, runs some isr, and
> >> then returns to spinning. Implementations of spinlocks could block all
> >> interrupts and never allow the processor to do anything other than busy
> >> wait for the lock. Such implementations are generally considered
> >> deficient as they block management of timers and TLBs and other
> critical
> >> global OS resources while not providing any added benefit. You are of
> >> course free to implement this sort of deficient simplistic private
> >> spinlock, but I would suggest not deploying this spinlock of yours on
> >> general purpose multiprocessor systems.
> >>
> >>
> >>
> >>
> >>
> >> –
> >> Mark Roddy
> >> — NTDEV is sponsored by OSR For our schedule of WDF, WDM, debugging
> >> and other seminars visit: http://www.osr.com/seminars To unsubscribe,
> >> visit the List Server section of OSR Online at
> >> http://www.osronline.com/page.cfm?name=ListServer
> >> —
> >> NTDEV is sponsored by OSR
> >>
> >> For our schedule of WDF, WDM, debugging and other seminars visit:
> >> http://www.osr.com/seminars
> >>
> >> To unsubscribe, visit the List Server section of OSR Online at
> >> http://www.osronline.com/page.cfm?name=ListServer
> >
> >
> >
> > —
> > NTDEV is sponsored by OSR
> >
> > For our schedule of WDF, WDM, debugging and other seminars visit:
> > http://www.osr.com/seminars
> >
> > To unsubscribe, visit the List Server section of OSR Online at
> > http://www.osronline.com/page.cfm?name=ListServer
>
>
> —
> NTDEV is sponsored by OSR
>
> For our schedule of WDF, WDM, debugging and other seminars visit:
> http://www.osr.com/seminars
>
> To unsubscribe, visit the List Server section of OSR Online at
> http://www.osronline.com/page.cfm?name=ListServer
>
–
Mark Roddy