On Wednesday 14 November 2007 19:37, David Brownell wrote:
> On Tuesday 13 November 2007, Nick Piggin wrote:
> > Upstream, all spinlocks prevent preemption.
>
> I chose my wording carefully though. A preemption point is
> more than just a small region where preemption isn't allowed.
>
> It's one of those where preemption is *INVITED* ...
With CONFIG_PREEMPT upstream, that's exactly the same (unless
you're considering preempt breaking points, which you don't
seem t obe).
> Now, in the RT case, I believe the rationale for inviting
> preemption when dropping a lock is largely related to the
> way priority inversion is handled. When lock contention can
> block higher priority activities, dropping the lock must
> be able to trigger the relevant activity switch.
There is no specific inviting of preemption. The locks are
preemptible -- they can be preempted even while being *held*
> ... and the raw spinlocks don't support that machinery,
> while "normal" spinlocks become inversion-aware mutexes.
>
> > But these ones
> > are raw locks rather than normal locks probably because that
> > they are trivially an innermost and correct lock.
>
> As in the $SUBJECT case, I'd say.
>
> Although another point is related to "trivial": the data
> is being protected through an operation too trivial to be
> worth paying for any of that priority logic.
A driver shouldn't get to decide that, IMO. And if there is
some policy in the -rt tree allowing these decisions, then
it's exactly the kind of thing we don't want upsream.
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