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    SubjectRe: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support
    Date
    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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