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    SubjectRe: [Ksummit-2010-discuss] [v2] Remaining BKL users, what to do
    On Tue, Oct 19, 2010 at 08:39:58AM -0400, Steven Rostedt wrote:
    > On Tue, 2010-10-19 at 09:26 +0200, Arnd Bergmann wrote:
    > > On Tuesday 19 October 2010 06:52:32 Dave Airlie wrote:
    > > > > I might be able to find some hardware still lying around here that uses an
    > > > > i810. Not sure unless I go hunting it. But I get the impression that if
    > > > > the kernel is a single-CPU kernel there is not any problem anyway? Don't
    > > > > distros offer a non-smp kernel as an installation option in case the user
    > > > > needs it? So in reality how big a problem is this?
    > > >
    > > > Not anymore, which is my old point of making a fuss. Nowadays in the
    > > > modern distro world, we supply a single kernel that can at runtime
    > > > decide if its running on SMP or UP and rewrite the text section
    > > > appropriately with locks etc. Its like magic, and something like
    > > > marking drivers as BROKEN_ON_SMP at compile time is really wrong when
    > > > what you want now is a runtime warning if someone tries to hotplug a
    > > > CPU with a known iffy driver loaded or if someone tries to load the
    > > > driver when we are already in SMP mode.
    > >
    > > We could make the driver run-time non-SMP by adding
    > >
    > > if (num_present_cpus() > 1) {
    > > pr_err("i810 no longer supports SMP\n");
    > > return -EINVAL;
    > > }
    > >
    > > to the init function. That would cover the vast majority of the
    > > users of i810 hardware, I guess.
    >
    > I think we also need to cover the PREEMPT case too. But that could be a
    > compile time check, since you can't boot a preempt kernel and make it
    > non preempt.
    >
    There are enough nameless embedded vendors that have turned a preempt
    kernel in to a non-preempt one at run-time by leaking the preempt count,
    whether by design or not, so it's certainly possile :-)


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