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    SubjectRe: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of cpu_relax for Blackfin SMP like patch
    On Wed, 11 Mar 2009 17:50:21 +0800 graff yang <graff.yang@gmail.com> wrote:

    > On Wed, Mar 11, 2009 at 12:48 AM, Andrew Morton
    > <akpm@linux-foundation.org> wrote:
    > >
    > > On Tue, 10 Mar 2009 06:25:08 -0400 Mike Frysinger <vapier.adi@gmail.com> wrote:
    > >
    > > > On Tue, Mar 10, 2009 at 06:07, gyang wrote:
    > > > > On Fri, 2009-03-06 at 14:37 -0800, Andrew Morton wrote:
    > > > >> On Fri, __6 Mar 2009 14:42:44 +0800
    > > > >> Bryan Wu <cooloney@kernel.org> wrote:
    > > > >>
    > > > >> > From: Graf Yang <graf.yang@analog.com>
    > > > >> >
    > > > >> > We are making a SMP like patch to blackfin, cpu_relax() is replaced by a
    > > > >> > data cache flush function which will count it to a per-cpu counter.
    > > > >> > If this serial function is called too early, the per-cpu data area have
    > > > >> > not been initialized, this call will cause crash.
    > > > >>
    > > > >> That's a bug in blackfin architecture support. __The kernel should be
    > > > >> able to call cpu_relax() at any time, surely. __It's a very low-level
    > > > >> and simple thing.
    > > > >>
    > > > >> > So we'd like to use barrier() instead of cpu_relax().
    > > > >> >
    > > > >>
    > > > >> barrier() is purely a compiler concept. __We might as well just remove
    > > > >> the cpu_relax() altogether.
    > > > >
    > > > > Do you mean remove cpu_relax(), and either not add barrier() here?
    > > >
    > > > afaik, early printk all runs before SMP is setup, so having it be a
    > > > 100% busy wait is fine
    > >
    > > No, blackfin is busted, please fix this bug in blackfin core.
    > >
    > > What happens if core kernel code decides to run cpu_relax() prior to
    > > initialising per-cpu data?
    >
    > cpu_relax() will call smp_mb(), and it need to invalidate data-cache
    > in blackfin smp-like kernel,
    > the cache flush number is increased and recorded into a per-cpu data.
    >
    > When cpu_relax() is called from early-printk functions, the per-cpu
    > data areas have not
    > been initialized.

    Right. And we've demonstrated here that this was a bad idea.

    So I suggest that the blackfin maintainers delete that code.

    If it's really valuable (which I doubt) then it could be reimplemented
    via a statically-allocated array of longs and local_irq_save()
    protection. Or a statically allocated array of atomic_long_t's.



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